<PAGE>   1
                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549

Form 10-Q
---------


[X]  QUARTERLY REPORT PURSUANT TO SECTION 13 or 15(d) OF THE
     SECURITIES EXCHANGE ACT Of 1934

     For the quarterly period ended       March 31, 1999
                                    --------------------------
    
[ ]  TRANSITION REPORT PURSUANT TO SECTION 13 or 15(d) OF THE
     SECURITIES EXCHANGE ACT Of 1934

     For the transition period from                 to
                                    ---------------    ---------------

Commission File Number     0-15902
                       ---------------------------------------------------------

                                Essef Corporation
--------------------------------------------------------------------------------
             (Exact name of registrant as specified in its charter)


             Ohio                                                 34-0777631
--------------------------------------------------------------------------------
(State or other jurisdiction of                               (I.R.S. Employer
 incorporation or organization)                              Identification No.)


    220 Park Drive, Chardon, Ohio                                   44024
--------------------------------------------------------------------------------
(Address of principal executive offices)                          (Zip Code)


Registrant's telephone number, including area code  (440) 286-2200
                                                   ----------------

                                      None
--------------------------------------------------------------------------------
Former name, former address and former fiscal year, if changed since last
report.


Indicate by check mark whether the registrant (1) has filed all reports required
to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934, as
amended, during the preceding 12 months (or for such shorter period that the
registrant was required to file such reports), and (2) has been subject to such
filing requirements for the past 90 days. Yes  X   No       N/A
                                              ---      ---       ---

Indicate the number of shares outstanding of each of the issuer's classes of
common shares, as of the latest practicable date.


            Class                     Outstanding at May 10,1999
--------------------------------------------------------------------------------
Common Shares, no par value              13,073,400 Shares



                                  Page 1 of 18

<PAGE>   2

                                ESSEF CORPORATION
                                    FORM 10-Q

                        FOR QUARTER ENDED MARCH 31, 1999

                                      INDEX

                                                                      Sequential
                                                                       Page No.
                                                                       --------

Part I - Financial Information

     Item 1.  Financial Statements
          Condensed Consolidated Balance Sheets -
            March 31, 1999 and September 30, 1998...................          3
          Condensed Consolidated Statements of Income -
            Three Months and Six Months Ended March 31, 1999
            and 1998.................................................          4
          Condensed Consolidated Statements of Cash Flows -
            Six Months Ended March 31, 1999 and 1998.................          5
          Notes to Condensed Consolidated Financial
            Statements...............................................       6-10

     Item 2.  Management's Discussion and Analysis of
                Financial Condition and Results of Operations........      11-14

Part II - Other Information

     Item 1.  Legal Proceedings......................................         15

     Item 2.  Changes in Securities..................................         15

     Item 4.  Submission of Matters to a Vote of Security
                Holders..............................................         15

     Item 6.  Exhibits and Reports on Form 8-K.......................         15



                                  Page 2 of 18

<PAGE>   3


PART I.  FINANCIAL INFORMATION

ITEM 1.  FINANCIAL STATEMENTS

                       ESSEF CORPORATION AND SUBSIDIARIES
                      CONDENSED CONSOLIDATED BALANCE SHEETS
                             (Dollars in thousands)

<TABLE>
<CAPTION>
                                                         
                                                         March 31,         September 30,                      
ASSETS                                                     1999                1998
------                                                  -----------         ---------
                                                        (unaudited)         (audited)
<S>                                                     <C>                 <C>
Current Assets
      Cash and cash equivalents ...............          $  2,271           $  1,137
      Accounts receivable, net ................            90,405             44,545
      Inventories, net ........................            53,004             50,163
        Prepayments and other .................             5,634              3,069
                                                         --------           --------
           Total current assets ...............           151,314             98,914

Property, plant and equipment, net ............            77,332             79,076
Goodwill, net .................................            74,163             74,444
Deferred income taxes .........................             3,415              4,391
Other .........................................             7,770             10,098
                                                         --------           --------
                                                         $313,994           $266,923
                                                         ========           ========
LIABILITIES AND SHAREHOLDERS' EQUITY
------------------------------------

Current Liabilities
        Short-term borrowings .................          $  5,398           $  3,853
        Current maturities of long-term debt ..               662                859
        Accounts payable ......................            25,903             23,871
        Accrued expenses ......................            36,797             33,301
        Accrued income taxes ..................             3,592              3,685
                                                         --------           --------
           Total current liabilities ..........            72,352             65,569

Long-term debt ................................           151,554            112,622
Other long-term Liabilities ...................             6,356              5,054

Shareholders' Equity
        Preferred shares without par value,
           authorized 1,000,000 shares,
           none issued ........................
                                                         --------           --------
        Common shares without par value,
           authorized 40,000,000 shares, issued
           13,687,425 and 13,503,734 shares, ..            52,192             50,570
        Treasury shares at cost, 618,127
           and 503,927 shares, respectively ...            (9,898)            (7,962)
        Retained earnings .....................            41,994             40,888
        Other Comprehensive Income ............              (556)               182
           Total shareholders' equity .........            83,732             83,678
                                                         --------           --------
                                                         $313,994           $266,923
                                                         ========           ========
</TABLE>


See notes to condensed consolidated financial statements.


                                  Page 3 of 18



<PAGE>   4


                       ESSEF CORPORATION AND SUBSIDIARIES
                   CONDENSED CONSOLIDATED STATEMENTS OF INCOME
                      (In thousands, except per share data)
                                   (Unaudited)


                                  Three Months Ended      Six Months Ended
                                       March 31,              March 31,
                                   1999        1998        1999        1998 
                                  ------     -------    --------      ------

Net sales....................   $115,202    $ 91,756    $208,822   $ 171,374

Cost of sales................     84,692      66,621     155,302     128,108
                                --------    --------    --------    --------

  Gross profit...............     30,510      25,135      53,520      43,266

Operating expenses...........     24,383      19,894      47,186      36,291
                                --------    --------    --------    --------

  Income from operations.....      6,127       5,241       6,334       6,975

Interest and other expense...      2,499       2,157       4,632       3,765 
                                --------    --------    --------    --------

  Income before income taxes.      3,628       3,084       1,702       3,210

Provision for income taxes...      1,270       1,079         596       1,123
                                --------    --------    --------    --------

  Net income.................   $  2,358    $  2,005    $  1,106    $  2,087
                                ========    ========    ========    ========

Earnings per share:

  Basic                             $.18        $.16        $.08        $.16
                                ========    ========    ========    ========

  Diluted                           $.16        $.14        $.07        $.14 
                                ========    ========    ========    ========


Average shares outstanding:

  Basic                           13,078      12,885      13,065      12,850
                                ========    ========    ========    ========

  Diluted                         15,001      14,786      14,967      14,769
                                ========    ========    ========    ========


See notes to condensed consolidated financial statements.


                                  Page 4 of 18

<PAGE>   5



                       ESSEF CORPORATION AND SUBSIDIARIES
                 CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS
                                 (In thousands)
                                   (Unaudited)

<TABLE>
<CAPTION>
                                                                     Six Months Ended
                                                                         March 31,
                                                                  1999               1998 
                                                                --------           --------
<S>                                                             <C>                <C>
Cash Flows from Operating Activities
      Net income .....................................          $  1,106           $  2,087
      Adjustments to reconcile net income
      to net cash used for operating activities
                Depreciation and amortization ........             7,418              5,413
                Deferred taxes .......................             1,043                308
                Other ................................               538               (761)
      Changes in operating assets and liabilities
           Accounts receivable .......................           (46,864)           (33,245)
           Inventories ...............................            (3,377)            (8,956)
           Prepayments and other .....................            (2,610)              (216)
           Accounts payable ..........................             2,512              9,945
           Accrued expenses ..........................             5,950             (5,698)
           Accrued income taxes ......................               (93)            (1,088)
                                                                --------           --------
                Net cash used in operating activities            (34,377)           (32,211)
                                                                --------           --------

Cash Flows for Investing Activities
      Additions to property, plant and equipment .....            (4,717)            (9,625)
      Proceeds on sale of land and buildings .........             1,127                 --
      Business acquisitions ..........................              (412)            (5,117)
      Other, net .....................................                --             (2,169)
                                                                --------           --------
                Net cash used in investing activities             (4,002)           (16,911)
                                                                --------           --------

Cash Flows from Financing Activities
      Proceeds from long term debt ...................            39,060             49,013
      Increase in short-term borrowings ..............             1,220                 54
      Treasury stock acquired ........................            (1,936)                --
      Proceeds from exercise of stock options ........             1,169                188
                                                                --------           --------
           Net cash provided by financing activities .            39,513             49,255
                                                                --------           --------

Net change in cash and cash equivalents ..............             1,134                133

Cash and cash equivalents
  Beginning of period ................................             1,137              1,668
                                                                --------           --------
  End of period ......................................          $  2,271           $  1,801
                                                                ========           ========


Supplemental Cash Flow Information
           Interest paid .............................          $  4,467           $  3,414
                                                                ========           ========
           Income taxes paid .........................          $    165           $  2,206
                                                                ========           ========
</TABLE>


See notes to condensed consolidated financial statements.



                                  Page 5 of 18



<PAGE>   6


                       ESSEF CORPORATION AND SUBSIDIARIES
              NOTES TO CONDENSED CONSOLIDATED FINANCIAL STATEMENTS


(1)        The accompanying unaudited condensed consolidated financial
           statements contain all adjustments (consisting of only normal and
           recurring adjustments) which, in the opinion of management, are
           necessary to present fairly the consolidated financial position of
           Essef Corporation and subsidiaries (the "Company") as of March 31,
           1999, and the results of operations for the three month and six month
           periods ended March 31, 1999 and 1998, and cash flows for the
           six-month periods ended March 31, 1999 and 1998.

           These condensed consolidated financial statements should be read in
           conjunction with the consolidated financial statements and the notes
           thereto included in the Company's 1998 Annual Report to Shareholders,
           sections of which are incorporated into the Company's Form 10-K filed
           for the fiscal year ended September 30, 1998. The results of
           operations for the three month and six month periods ended March 31,
           1999 may not necessarily be indicative of the operating results for
           the full year.

(2)        RECLASSIFICATIONS

           Certain reclassifications have been made to prior year amounts in
           order to be consistent with the presentation for the current year.

(3)        EARNINGS PER SHARE

           Earnings per share is computed in accordance with Statement of
           Financial Accounting Standards No. 128, "Earnings Per Share". Basic
           earnings per share is computed by dividing net income by the weighted
           average number of common shares outstanding during the period.
           Diluted earnings per share is based on the combined weighted average
           number of shares outstanding which include the assumed exercise or
           conversion of options. In computing diluted earnings per share, the
           Company has utilized the treasury stock method.

           The computation of weighted average common and common equivalent
           shares used in the calculation of basic and diluted earnings per
           share is as follows:



                                  Page 6 of 18


<PAGE>   7

<TABLE>
                                           Three Months Ended        Six Months Ended
                                                March 31,               March 31,
                                            1999        1998        1999        1998 
                                           -------     -------     -------     -------
                                               (unaudited)            (unaudited)
             <S>                           <C>         <C>         <C>         <C>
             Numerator
              Net income available to
               common shareholders         $ 2,358     $ 2,005     $ 1,106     $ 2,087
                                           =======     =======     =======     =======
             Denominator
              Weighted average common
               shares outstanding           13,078      12,885      13,065      12,850

              Dilutive effect of
               stock options                 1,923       1,901       1,902       1,919
                                           -------     -------     -------     -------
             Denominator for net
              income per diluted share      15,001      14,786      14,967      14,769

             Earnings per share:

              Basic                           $.18        $.16        $.08        $.16
                                           =======     =======     =======     =======

              Diluted                         $.16        $.14        $.07        $.14
                                           =======     =======     =======     =======
</TABLE>



 (4)         INVENTORIES

             Inventories are valued as follows:
                  (Dollars in thousands)
<TABLE>
<CAPTION>
                                                        March 31,            September 30,
                                                          1999                   1998  
                                                       -----------            ---------
                                                       (unaudited)            (audited)
                <S>                                    <C>                    <C>
                FIFO COST
                  Raw materials......................    $23,501                $24,543
                  Work-in-process....................      1,694                  1,926
                  Finished goods.....................     29,009                 24,818
                                                         -------                -------
                                                          54,204                 51,287
                    Excess of FIFO over LIFO cost....     (1,200)                (1,124)
                                                         -------                -------
                       Net Inventories...............    $53,004                $50,163
                                                         =======                =======
</TABLE>

(5)          SHORT-TERM BORROWINGS

             The Company's European subsidiaries have working capital lines of
             credit of approximately $12,000,000. At March 31, 1999 and
             September 30, 1998, $2,987,000 and $1,595,000, respectively was
             outstanding. At March 31, 1999, interest was at rates ranging from
             3.95% to 9.0%. In addition, a note payable of $2,411,000 and
             $2,258,000 relating to an acquisition was outstanding at March 31,
             1999 and September 30, 1998, respectively. At March 31, 1999, the
             interest rate on the note was 6%.



                                  Page 7 of 18

<PAGE>   8


(6)          LONG-TERM DEBT

             The Company through its bank group has an unsecured $185,000,000
             multi-currency revolving loan facility ("Credit Facility"). The
             Credit Facility matures April 30, 2003 and may be extended in one
             year increments with the approval of the bank group. The Credit
             Facility includes commitment reductions at specified dates and for
             events throughout the term of the loan; however, the commitment
             does not reduce below $135,000,000. Interest rates are based on
             increments over the LIBOR or foreign currency equivalent rate. A
             17.5 basis point facility fee is payable on the total amount of the
             commitment. As of March 31, 1999, interest rates ranged from 5.26%
             to 5.62%. The Company is in compliance with all of its covenants
             under its credit facilities.

             In October 1998, the Company entered into an interest rate swap
             arrangement with one of the members of its bank group. The swap
             arrangement is for a $50,000,000 notional amount for a two-year
             term with a bank option for a third year and fixes the Company's
             interest rate at 4.45% plus the applicable bank margin based on the
             Company's leverage ratio. The effective interest rate on this
             portion of debt was 4.945% at March 31, 1999. Additionally, in May
             1997, the Company entered into an interest rate swap arrangement
             with another commercial bank for a $30,000,000 notional amount for
             a two-year period that effectively fixes the Company's interest
             rate at 6.33% plus the applicable bank margin based on the
             Company's leverage ratio. The effective interest rate on this
             portion of debt was 6.83% at March 31, 1999. The Company does not
             use derivatives for trading purposes.

             Long-term debt consists of the following:
             (In thousands)

<TABLE>
<CAPTION>
                                                                        March 31,             September 30,
                                                                          1999                    1998
                                                                        --------                --------
                                                                       (unaudited)              (audited)
                    <S>                                                <C>                      <C>
                    Revolving credit facilities                          $150,000               $110,940
                    Other                                                   2,216                  2,541
                                                                         --------               --------
                                                                          152,216                113,481
                    Less current maturities                                  (662)                  (859)
                                                                         --------               --------
                    Long-term debt                                       $151,554               $112,622
                                                                         ========               ========
</TABLE>

(7)          COMMON STOCK

             Under a stock repurchase program authorized by the Company's Board
             of Directors, the Company purchased 114,200 shares of treasury
             stock in the six month period ended March 31, 1999, for $1,936,000.



                                  Page 8 of 18


<PAGE>   9

(8)           COMPREHENSIVE INCOME

              Effective October 1, 1998, the Company adopted Statement of
              Financial Accounting Standard's No. 130 ("SFAS 130") "Reporting
              Comprehensive Income". SFAS 130 establishes new standards for
              reporting comprehensive income and its components. Comprehensive
              income is a measurement of all changes in shareholders' equity
              that result from transactions and other economic events other than
              transactions with shareholders.

              Consolidated statements of comprehensive income are as follows:

<TABLE>
<CAPTION>
                                            Three Months Ended      Six Months Ended
                                                 March 31,               March 31,
                                             1999        1998        1999        1998 
                                            ------     -------    --------      ------
                                                (Unaudited)            (Unaudited)

               <S>                          <C>         <C>         <C>         <C>   
               Net Income                   $2,358      $2,005      $1,106      $2,087

               Other Comprehensive
                   (loss)/Income:
                Foreign Currency
                 translation adjustment       (879)       (172)       (738)         77
                                            ------      ------      ------      ------

              Comprehensive Income          $1,479      $1,833      $  368      $2,164
                                            ======      ======      ======      ======
</TABLE>

(9)           STOCK DIVIDEND

              On February 4, 1999, the Board of Directors authorized a 10%
              dividend which was distributed on March 10, 1999 to shareholders
              of record on February 19, 1999. The consolidated financial
              statements have been retroactively restated to reflect the number
              of shares outstanding following the dividend.


(10)          ACCOUNTING PRONOUNCEMENTS

              In June 1997, the Financial Accounting Standards Board ("Board")
              issued Statement of Financial Accounting Standard's No. 131 ("SFAS
              131") "Disclosures about Segments of an Enterprise and Related
              Information." SFAS 131 redefines how operating segments are
              determined and requires disclosure of certain financial and
              descriptive information about a company's operating segments.
              Applying SFAS 131, the Company would presently have three
              reportable segments: Swimming Pool and Spa Equipment, Water
              Treatment and Systems Equipment and Swimming Pool Sales and
              Installation. The Company will adopt SFAS 131 for its financial
              statement disclosures in its annual report for the year ending
              September 30, 1999.



                                  Page 9 of 18



<PAGE>   10




(11)         LITIGATION

             There has been no material change to the status of the litigation
             referred to in the Company's 1998 Annual Report to Shareholders,
             sections of which are incorporated in the Company's Form 10-K filed
             for the fiscal year ended September 30, 1998.


(12)         SUBSEQUENT EVENT

             On April 30, 1999, the Company announced that it had entered
             into a merger agreement with Pentair, Inc. whereby each Essef
             common share would be exchanged for $19.09 cash. In addition
             to the $19.09 in cash per share, just prior to the merger,
             the Company will distribute to its shareholders, for each
             share of Essef common stock held, 0.25 shares of its Anthony
             & Sylvan Pools Corporation subsidiary, thus creating a new
             stand-alone public company. Holders of Essef shares
             immediately prior to the consummation of the acquisition will
             receive Anthony & Sylvan shares in connection with the
             split-off. Based on independent valuation analyses, the value
             of the Anthony & Sylvan stock to be received by Essef
             shareholders is expected to be $3.00 - $3.50 for each Essef
             share held. The purchase price for the Essef common shares
             will total approximately $312 million in cash plus the
             assumption of debt. These transactions are expected to close
             on or about July 31, 1999.

             The merger agreement, which was approved by the boards of directors
             of both Essef and Pentair, is subject to Essef shareholder
             approval, regulatory approval under the Hart-Scott-Rodino Act and
             the completion of limited due diligence by Pentair. The
             distribution of Anthony & Sylvan shares is subject to the clearance
             of a registration statement to be filed with the Securities and
             Exchange Commission. The receipt of Anthony & Sylvan shares will be
             treated as a taxable distribution to Essef shareholder's, and will
             not be preceded by an initial public offering as earlier announced
             by Essef in 1998. Anthony & Sylvan, as part of the transaction with
             Pentair, will also assume $17 million of Essef debt, subject to
             certain adjustments.



                                  Page 10 of 18


<PAGE>   11




ITEM 2.


    MANAGEMENT'S DISCUSSION AND ANALYSIS OF FINANCIAL CONDITION AND RESULTS
                                 OF OPERATIONS

                 THREE MONTHS ENDED MARCH 31, 1999 COMPARED WITH
                        THREE MONTHS ENDED MARCH 31, 1998


Net sales of $115,202,000 for the second quarter of fiscal 1999 increased 26%
over fiscal 1998 net sales of $91,756,000. The increase was primarily
attributable to the Swimming Pool and Spa Equipment Segment and the Swimming
Pool Sales and Installation Segment as a result of four acquisitions completed
in the latter part of fiscal 1998.

Gross profit decreased from 27.4% to 26.5%. The decrease was primarily
attributable to increased sales from the Swimming Pool Sales and Installation
Segment which has lower gross margin's during it's off-season than the Company's
other businesses. Additionally, employee and other related costs incurred in the
Water Treatment and Systems Equipment Segment related to staffing reductions
also contributed to the decrease for the quarter. The decrease was partially
offset by improved gross profit margins in the Swimming Pool and Spa Equipment
Segment as a result of increased sales volumes and acquisitions.

Operating expenses, consisting of engineering and development, selling, and
administrative expenses, as a percentage of sales decreased from 21.7% to 21.2%.
Reductions within the Swimming Pool and Spa Equipment Segment due to synergies
created with prior year acquisitions were partially offset by increases within
the Swimming Pool Sales and Installation Segment which were attributable to
increased spending for sales and marketing lead-generating activities.

Interest and other expense increased by $342,000 to $2,499,000. The increase was
the result of increased borrowings, which were used to finance the acquisitions
completed in fiscal 1998.

The Company's effective tax rate was 35% in both periods.

As a result of the above items, net income of $2,358,000 or $.16 per diluted
share, increased $353,000 from $2,005,000 or $.14 per diluted share in the prior
year.

                  SIX MONTHS ENDED MARCH 31, 1999 COMPARED WITH
                         SIX MONTHS ENDED MARCH 31, 1998

Net sales of $208,822,000 increased 22% over fiscal 1998 net sales of
$171,374,000. As with the second quarter results described above, the increase
was attributable to five acquisitions completed in fiscal 1998 which benefited
both the Swimming Pool and Spa Equipment and Swimming Pool Sales and
Installation Segments.



                                  Page 11 of 18


<PAGE>   12

Gross profit increased from 25.2% of sales to 25.6% as a result of improved
gross profit margins within the Swimming Pool and Spa Equipment Segment. The
increase in gross margins was partially offset by an increase in sales within
the Swimming Pool Sales and Installation Segment which has lower gross margins
than the Company's other businesses.

Operating expenses, consisting of engineering and development, selling, and
administrative expenses, as a percentage of sales increased from 21.2% to 22.6%.
The increase came principally from the Swimming Pool Sales and Installation
Segment due to additional sales and marketing lead-generating activities and
lower than normal expenses in 1998 due to an adjustment in the accrual for
incentive based employee stock options related to a reduction in the number of
eligible stock options. Partially offsetting these increases were reductions in
the Water Treatment and Systems Equipment Segment as a result of a number of
cost reduction initiatives implemented during the first quarter of fiscal 1999.

Interest and other expense increased by $867,000 to $4,632,000. The increase was
the result of increased borrowings, which were used to finance the five
acquisitions completed in fiscal 1998.

The Company's effective tax rate was 35% in both periods.

As a result of the above items, net income of $1,106,000 or $.07 per diluted
share, decreased $981,000 from $2,087,000 or $.14 per diluted share in the prior
year period.


                         LIQUIDITY AND CAPITAL RESOURCES

At March 31, 1999 funded debt was $157,614,000 an increase of $40,280,000 from
September 30, 1998. The increase came principally as a result of normal seasonal
working capital requirements which increased working capital by $46,965,000 from
September 30, 1998 to a level of $85,022,000 at March 31, 1999.

Capital expenditures of $4,717,000 for the six months decreased $4,908,000
compared to $9,625,000 for the same period last year. The decrease is
attributable to higher than normal capital expenditures in fiscal 1998, related
to the Company's expansion into India and consolidation activities undertaken in
the Swimming Pool and Spa Equipment Segment following the acquisition of General
Aquatics and subsequent closure of the Company's City of Industry plant.

The Company believes that funds available under its revolving credit facility
and funds generated from operations will be sufficient to satisfy its
anticipated operating needs and capital improvements in fiscal 1999.



                                  Page 12 of 18

<PAGE>   13

The Company is involved in various claims and lawsuits incidental to its
business, including product liability claims which are covered by insurance
after certain deductibles. Although, the Company believes that its reserves are
adequate, a significant increase in the aggregate amount of claims could have an
adverse effect on the deductible level or upon the Company's ability to obtain
product liability coverage for certain product lines. While the ultimate result
of these contingencies cannot be predicted with certainty, management does not
expect these matters to have a material adverse effect on the consolidated
financial position, results of operations or cash flows of the Company.

                                YEAR 2000 MATTERS

The Company utilizes computer technologies to varying degrees throughout its
three business segments to effectively carry out its day-to-day operations.
Computer technologies include both information technology in the form of
hardware and software, as well as embedded technology in the Company's
facilities and equipment. Similar to most companies, the Company must determine
whether its systems are capable of recognizing and processing date sensitive
information properly as the year 2000 approaches.

During the past several years as a result of an operational decision to improve
its information systems, most of the significant business units within the Water
Treatment and Systems Equipment Segment and Swimming Pool and Spa Equipment
Segment have implemented enterprise-wide information systems. These information
systems are substantially year 2000 compliant, and therefore the costs to make
them fully compliant are not expected to be material. The Company is currently
in the process of assessing the information systems in the remaining business
units within these segments for year 2000 compliance. Business units whose
information systems are not year 2000 compliant will either implement the
Company's enterprise-wide information system or make modifications to existing
systems. In addition, these segments are in the process of evaluating the
computer controlled equipment and other systems utilized in the business that
use embedded processors that have the potential to be date sensitive to the year
2000 issue. Costs to replace or modify these information systems and processors
are not expected to exceed $1 million and will be completed by mid-1999. In
addition, the Swimming Pool Sales and Installation Segment in 1997, as part of
an overall modernization and upgrade of its information systems, began preparing
its computer systems and applications for the date change in the year 2000. To
date, this process has involved modifying or replacing certain hardware and
upgrading certain software and has not involved material costs to the Company.
The Company believes that substantially all of the necessary systems and
applications changes will be completed by mid-1999, and that the amount of
additional costs, if any, needed to address the year 2000 issue will be
immaterial. For all of the Company's business segments, the costs of year 2000
issue are being funded through operating cash flow and are not material to the
Company. The Company's segments have also not deferred any significant
information technology projects to address the year 2000 issue.



                                  Page 13 of 18


<PAGE>   14

The Company currently believes it will be able to modify, replace or mitigate
its affected systems in time to avoid any material detrimental impact on its
operations. If the Company determines that it may be unable to modify and
properly test affected systems on a timely basis, the Company intends to develop
appropriate contingency plans for any such mission-critical systems at the time
such determination is made. While the Company is not presently aware of any
significant exposure that its systems will not be properly changed or modified
on a timely basis, there can be no assurances that all year 2000 correction
activities will be completed and properly tested before the year 2000, or that
contingency plans will sufficiently mitigate the risk of a year 2000 readiness
problem. An interruption of the Company's ability to conduct its business due to
a year 2000 issue could have a material adverse effect on the Company.

The Company is in the process of surveying suppliers and customers critical to
its business for the purpose of obtaining assurance regarding their ability to
properly operate their systems in the year 2000. The Company has targeted the
completion of these activities by mid-1999. The Company will develop appropriate
contingency plans in the event that a significant exposure is identified
relative to the dependencies on third-party systems. While the Company is not
presently aware of any such significant exposure, there can be no guarantee that
the systems of third-parties on which the Company relies will be converted in a
timely manner, or that a failure to properly convert by another company would
not have a material adverse effect on the Company.

                    PRIVATE SECURITIES LITIGATION REFORM ACT

The Private Securities Litigation Reform Act of 1995 provides a "safe harbor"
for forward-looking statements. Certain information included in this report and
other materials filed with the Securities and Exchange Commission (as well as
information included in oral or other written statements made or to be made by
the Company) contains statements that are forward-looking. Such statements may
relate to plans for future expansion, plant integration's and consolidations,
business development activities, other capital spending, financing or the
effects of regulation and competition. Such information involves important risks
and uncertainties that could significantly affect anticipated results in the
future and, accordingly, such results may differ from those expressed in any
forward-looking statements made by or on behalf of the Company. These risks and
uncertainties include, but are not limited to those relating to: the costs of
integrating acquired businesses; product development activities; dependence on
existing management; global economic and market conditions; the impact of
weather on the pool businesses; the unpredictability of the stock markets;
events that may impact the availability of bank financing and year 2000 issues.


                                  Page 14 of 18


<PAGE>   15




PART II. OTHER INFORMATION

ITEM 1.  LEGAL PROCEEDINGS

         There has been no material change to the status of the legal 
         proceedings referred to in the 1998 Form 10-K during the period
         covered by this report.


ITEM 2.  CHANGES IN SECURITIES

         No change.


ITEM 4.  SUBMISSION OF MATTERS TO A VOTE OF THE SECURITY HOLDERS

         None


ITEM 6.  EXHIBITS AND REPORTS ON FORM 8-K

         (a)  Exhibits

              13     Independent Public Accountants' Review
                     Report

              15     Independent Public Accountants'
                     Awareness Letter

              27     Financial Data Schedule


         (b)  Form 8-K

              The following report was filed on Form 8-K during the quarter for 
              which this Report is filed

              Filed May 7, 1999 - Agreement and Plan of Merger dated April 30, 
              1999, among Pentair, Inc., Northstar Acquisition Company and 
              Essef Corporation.



                                  Page 15 of 18

<PAGE>   16


                                   SIGNATURE

Pursuant to the requirements of the Securities Exchange Act of 1934, the
Registrant has duly caused this report to be signed on its behalf by the
undersigned, thereunto duly authorized.



                                            ESSEF Corporation
                                            (Registrant)




                                            Thomas B. Waldin
                                            ------------------------------------
                                            THOMAS B. WALDIN
                                            President and
                                            Chief Executive Officer
                                            (Principal Executive Officer)




                                            Stuart D. Neidus
                                            ------------------------------------
                                            STUART D. NEIDUS
                                            Executive Vice President
                                            and Chief Financial Officer
                                            (Principal Accounting Officer)






Date: May 12, 1999




                                  Page 16 of 18