SRAC Publication No. 464

II

Southern

Regional

Aquaculture

Center

December 1992

Interactions of pH, Carbon Dioxide,

Alkalinity and Hardness in Fish Ponds

William A. Wurts and Robert M. Durborow*

Water quality in fish ponds is affected by the interactions of several chemical components. Carbon

dioxide, pH, alkalinity and hardness are interrelated and can have

profound effects on pond productivity, the level of stress and fish

health, oxygen availability and the

toxicity of ammonia as well as that

of certain metals. Most features of

water quality are not constant.

Carbon dioxide and pH concentrations fluctuate or cycle daily. Alkalinity and hardness are relatively

stable but can change over time,

usually weeks to months, depending on the pH or mineral content

of watershed and bottom soils.

pH and carbon dioxide

The measure which indicates

whether water is acidic or basic is

known as pH. More precisely, pH

indicates the hydrogen ion concentration in water and is defined as

the negative logarithm of the

molar hydrogen ion concentration

(-log [H+]). Water is considered

acidic when pH is below 7 and

basic when pH is above 7. Most

pH values encountered fall between O and 14. The recommended pH range for aquaculture

is 6.5 to 9.0 (Figure 1).

Fish and other vertebrates have an

average blood pH of 7.4. Fish

* Kentucky State University

blood comes into close contact

with water (1- or 2-cell separation)

as it passes through the blood vessels of the gills and skin. A desirable range for pond water pH

would be close to that of fish

blood (i.e., 7.0 to 8.0). Fish may become stressed and die if the pH

drops below 5 (e.g., acidic runoff)

or rises above 10 (e.g., low alkalinity combined with intense photosynthesis by dense algal blooms –

phytoplankton or filamentous

algae).

Pond pH varies throughout the

day due to respiration and photosynthesis. After sunset, dissolved

oxygen (DO) concentrations decline as photosynthesis stops and

all plants and animals in the pond

consume oxygen (respiration). In

heavily stocked fish ponds, carbon

dioxide (CO2

) concentrations can

become high as a result of respiration. The free CO2 released during

respiration reacts with water, producing carbonic acid (H2CO3

), and

pH is lowered.

H2O + CO2 = H2CO3 = H+ + HCO3

-

Table 1 summarizes the relative

changes in dissolved oxygen, CO2

and pH over 24 hours.

Carbon dioxide rarely causes direct toxicity to fish. However,

high concentrations lower pond

pH and limit the capacity of fish

blood to carry oxygen by lowering

blood pH at the gills. At a given

dissolved oxygen concentration

(e.g., 2 mg/L, milligrams per liter;

same as parts per million, ppm),

fish may suffocate when CO2 levels are high and appear unaffected

when CO2 is low. Catfish can tolerate 20 to 30 mg/L CO2 if accumulation is slow and dissolved

Figure 1. pH scale showing recommended range.
Table 1. Relative concentration changes for dissolved oxygen,

carbon dioxide and pH in ponds over 24 hours.

Change

Dissolved Carbon

Time Oxygen Dioxide pH

Daylight Increases Decreases Increases

Nightime Decreases Increases Decreases

Tucker (1984).

oxygen concentrations are above 5

mg/L. In a reservoir or natural

pond, CO2 rarely exceeds 5 to 10

mg/ L.

High CO2 concentrations are almost always accompanied by low

dissolved oxygen concentrations

(high respiration); the aeration

used to increase low dissolved oxygen will, to some extent, help reduce excess CO2 by improving its

diffusion back into the atmosphere. Chronically high CO2 levels can be treated chemically with

hydrated lime, Ca(OH)2

. Approximately 1 mg/L of hydrated lime

will remove 1 mg/L of CO2

. This

treatment should not be used in

waters with poor buffering capacity (low alkalinity) because pH

could rise to levels lethal to fish.

Also, fish could be endangered if

hydrated lime is added to waters

with high ammonia concentrations. High pH increases the toxicity of ammonia.

Alkalinity

The quantity of base present in

water defines what is known as

total alkalinity. Common bases

found in fish ponds include carbonates, bicarbonates, hydroxides,

phosphates and berates. Carbonates and bicarbonates are the most

common and most important components of alkalinity. Alkalinity is

measured by the amount of acid

(hydrogen ion) water can absorb

(buffer) before achieving a designated pH. Total alkalinity is expressed as milligrams per liter or

parts per million calcium carbonate (mg/L or ppm CaCO3

). A

total alkalinity of 20 mg/L or

more is necessary for good pond

productivity. A desirable range of

total alkalinity for fish culture is

between 75 and 200 mg/L CaCO3

.

Carbonate-bicarbonate alkalinity

(and hardness) in surface and well

waters is produced primarily

through the interactions of CO2

,

water and limestone. Rainwater is

naturally acidic because of exposure to atmospheric carbon dioxide. As rain falls to the earth, each

droplet becomes saturated with

CO2

; and pH is lowered. Well

water is pumped from large, natural underground reservoirs (aquifers) or small, localized pockets of

underground water (groundwater), Typically, underground

water has high CO2 concentrations, and low pH and oxygen concentrations. Carbon dioxide is

high in underground water because of bacterial processes in the

soils and various underground,

particulate mineral formations

through which water moves. As

ground- or rainwaters flow over

and percolate through soil and underground rock formations containing calcitic limestone (CaCO3

)

or dolomitic limestone

[CaMg(CO3

)2], the acidity produced by CO2 will dissolve limestone and form calcium and magnesium bicarbonate salts:

CaCO3 + H2O + CO2 = Ca+2 + 2HCO3

-

or

CaMg(CO3)2 + 2H2O + 2CO2 =

Ca+2 + Mg+2 + 4HCO3

-

The resultant water has increased

alkalinity, pH and hardness.

Alkalinity, pH and carbon

dioxide concentrations

In water with moderate to high alkalinity (good buffering capacity)

and similar hardness levels, pH is

neutral or slightly basic (7.0 to 8.3)

and does not fluctuate widely.

Higher amounts of CO2 (i.e., carbonic acid) or other acids are required to lower pH because there

is more base available to neutralize or buffer the acid. The relationTable 2. Factors for calculating carbon dioxide concentrations

in water with known pH, temperature and alkalinity

measurements. a

Temperatures (°C)

I

pH 5 10 15

6.0 2.915

6.2 1.839

6.4 1.160

6.6 0.732

6.8 0.462

7.0 0.291

7.2 0.184

7.4 0.116

7.6 0.073

7.8 0.046

8.0 0.029

8.2 0.018

8.4 0.012

2.539

1.602

1.010

0.637

0.402

0.254

0.160

0.101

0.064

0.040

0.025

0.016

0.010

2.315

1.460

0.921

0.582

0.367

0.232

0.146

0.092

0.058

0.037

0.023

0.015

0.009

20

2,112

1.333

0.841

0.531

0.335

0.211

0.133

0.084

0.053

0.034

0.021

0.013

0.008

25 30 35

1.970

1.244

0.784

0.495

0.313

0.197

0.124

0.078

0.050

0.031

0.020

0.012

0.008

1.882

1.187

0.749

0.473

0.298

0.188

0.119

0.075

0.047

0.030

0.019

0.012

0.008

1.839

1.160

0.732

0.462

0.291

0.184

0.116

0.073

0.046

0.030

0.018

0.011

0.007

Tucker (1984).

a

Factors should be multiplied by total alkalinity (mg/L) to get carbon dioxide (mg/L).

For practical purposes, CO2 concentrations are negligible above pH = 8.4.

2
ship among alkalinity, pH and

CO2 can be determined from Table

2. The number (factor) found in

the table which corresponds to the

measured pH and water temperature is multiplied by the measured

alkalinity value (mg/L as CaCO3

).

The product of these numbers estimates CO2 concentration (mg/L).

For example, Mr. Jacobs measures

a pH of 7.2, a temperature of 77°F

(25°C) and total alkalinity of 103

mg/L in his catfish pond. He determines the corresponding factor,

0.124, from Table 2 and multiplies

this number by the measured alkalinity, 103 mg/L. The resulting

product gives him an estimate of

the CO2 concentration in his pond:

0.1 24x 103 mg/L alkalinity =

12.8 mg/L CO2

A prompt pH measurement

within 30 minutes of water

sampling is required to minimize

error when using this method.

Due to several sources of error

that can occur with this method,

direct measurement of CO2 using

a chemical test procedure is preferred,

Alkalinity, pH and

photosynthesis

The bases associated with alkalinity react with and neutralize acids.

Carbonates and bicarbonates can

react with both acids and bases

and buffer (minimize) pH

changes. The pH of well buffered

water normally fluctuates between

6.5 and 9. In waters with low alkalinity, pH can reach dangerously

low (CO2 and carbonic acid from

high respiration) or dangerously

high (rapid photosynthesis) levels

(Figure 2).

Phytoplankton are microscopic or

near microscopic, aquatic plants

which are responsible for most of

the oxygen (photosynthesis) and

primary productivity in ponds. By

stabilizing pH at or above 6.5, alkalinity improves phytoplankton

productivity (pond fertility) by

increasing nutrient availability

(soluble phosphate concentrations). Alkalinities at or above 20

mg/L trap CO2 and increase the

pH

11

10

9

8

7

6

Early Mid Early

Morning Afternoon Morning

Figure 2. Changes in pH during a 24-hour period in waters of high and low

total alkalinities.

concentrations available for photosynthesis.

Because phytoplankton use CO2 in

photosynthesis, the pH of pond

water increases as carbonic acid

(i.e., CO2

) is removed. Also, phytoplankton and other plants can

combine bicarbonates (HCO3

-

) to

form CO2 for photosynthesis, and

carbonate (CO3

-2) is released:

2HCO3

- + phytoplankton =

CO2 (photosynthesis) + CO3

-2 + H2O)

CO3

-2 + H2O = HCO3- + OH-

(strong base)

High pH could also be viewed as a

decrease in hydrogen ions (H+):

CO3

-2 + H+ = HCO3- or

HCO3

- + H+ = H2O + CO2

The release of carbonate converted

from bicarbonate by plant life can

cause pH to climb dramatically

(above 9) during periods of rapid

photosynthesis by dense phytoplankton (algal) blooms. This rise

in pH can occur in low alkalinity

water (20 to 50 mg/L) or in water

with moderate to high bicarbonate

alkalinity (75 to 200 mg/L) that

has less than 25 mg/L hardness.

High bicarbonate alkalinity in soft

water is produced by sodium and

potassium carbonates which are

more soluble than the calcium and

magnesium carbonates that cause

3

hardness. If calcium, magnesium

and photosynthetically produced

carbonate are present when pH is

greater than 8.3, limestone is

formed. Ponds with alkalinities

below 20 mg/L do not usually

support good phytoplankton

blooms and do not commonly experience dramatic pH increases because of intense photosynthesis.

Hardness

Water hardness is important to

fish culture and is a commonly reported aspect of water quality. It

is a measure of the quantity of divalent ions (for this discussion,

salts with two positive charges)

such as calcium, magnesium

and/or iron in water. Hardness

can be a mixture of divalent salts;

however, calcium and magnesium

are the most common sources of

water hardness.

Hardness is traditionally measured by chemical titration. The

hardness of a water sample is reported in milligrams per liter as

calcium carbonate (mg/L CaCO3

).

Calcium carbonate hardness is a

general term that indicates the

total quantity of divalent salts present and does not specifically identify whether calcium, magnesium
and/or some other divalent salt is

causing water hardness.

Hardness is commonly confused

with alkalinity (the total concentration of base). The confusion relates to the term used to report

both measures, mg/L CaCO3

. If

limestone is responsible for both

hardness and alkalinity, the concentrations will be similar if not

identical. However, where sodium bicarbonate (NaHCO3

) is responsible for alkalinity it is possible to have low hardness and high

alkalinity. Acidic, ground or well

water can have low or high hardness and has little or no alkalinity.

Calcium and magnesium are essential in the biological processes

of fish (bone and scale formation,

blood clotting and other metabolic

reactions). Fish can absorb calcium and magnesium directly

from the water or from food.

However, calcium is the most important environmental, divalent

salt in fish culture water. The presence of free (ionic), calcium in culture water helps reduce the loss of

other salts (e.g., sodium and potassium) from fish body fluids (i.e.,

blood). Sodium and potassium

are the most important salts in fish

blood and are critical for normal

heart, nerve and muscle function.

Research has shown that environmental calcium is also required to

re-absorb these lost salts. In low

calcium water, fish can lose (leak)

substantial quantities of sodium

and potassium into the water.

Body energy is used to re-absorb

the lost salts. For some species

(e.g., red drum and striped bass),

relatively high concentrations of

calcium hardness are required for

survival.

A recommended range for free calcium in culture waters is 25 to 100

mg/L (63 to 250 mg/L CaCO3

hardness). Channel catfish can tolerate low calcium concentrations

as long as their feed contains a

minimum level of mineral calcium

but may grow slowly under these

conditions. Similarly, rainbow

trout can tolerate waters with free

calcium concentrations as low as

10 mg/L if pH is above 6.5. If

freshwater culture of striped bass,

red drum or crawfish is being considered, free calcium concentrations in the 40 to 100 mg/L range

(100 to 250 mg/L as CaCO3 hardness) are desirable; a value of 100

mg/L (250 mg/ L calcium hardness) matches the calcium concentration of fish blood. Tests specific

for calcium hardness should be

performed on samples of the

water source being considered for

these animals.

A low CaCO3 hardness value is a

reliable indication that the calcium

concentration is low. However,

high hardness does not necessarily

reflect a high calcium concentration. But, since limestone is common in the soil and bedrock of the

southern United States, it would

be reasonably safe to assume that

high hardness measurements reflect high calcium levels.

A CaCO3 hardness value of 100

mg/L represents a free calcium

concentration of 40 mg/L (divide

CaCO3 value by 2.5) if hardness is

caused by the presence of calcium

only. Similarly, a CaCO3 value of

100 mg/L represents a free magnesium value of 24 mg/L (divide

CaCO3 value by 4.12) if hardness

is caused by magnesium only.

These factors (2.5 and 4.12) are related to the molecular weight of

CaCO3 and the difference in

weights between calcium and magnesium atoms. Where hardness is

caused by limestone, the CaCO3

value usually reflects a mixture of

free calcium and magnesium with

calcium being the predominant divalent salt.

Agricultural limestone can be used

to increase calcium concentrations

(and carbonate-bicarbonate alkalinity) in areas with acid waters or

soils. However, at a pH of 8.3 or

greater, agricultural limestone will

not dissolve. Agricultural gypsum

(calcium sulfate) or food grade calcium chloride could be used to

raise calcium levels in soft, alkaline waters. Expense might be prohibitive if large volumes of water

need treatment. Identifying a suitable water source may be more

practical.

Effects of pH, alkalinity

and hardness on ammonia

and metal toxicities

Ammonia becomes more toxic as

pH increases. Higher concentrations of the toxic form of ammonia

(NH3

) are formed in basic waters;

while the less toxic form, ammonium (NH4

+

), is more prevalent in

acidic waters. Since alkalinity increases pH, ammonia will be more

toxic in waters with high total alkalinity. Hardness is not typically

associated with ammonia toxicity.

Metals such as copper and zinc are

commonly used around aquatic

environments (tanks, plumbing

and copper sulfate). These metals

become more soluble in acidic environments. The soluble or free

ionic forms of these metals are

toxic to fish. High total alkalinity

increases pH and available bases

which produce less toxic or insoluble forms of copper and zinc.

High concentrations of calcium

and magnesium (hardness) block

the effects of copper and zinc at

their sites of toxic action. Therefore, copper and zinc are more

toxic to fish in soft, acidic waters

with low total alkalinity.

Ideally, an aquaculture pond

should have a pH between 6.5 and

9 as well as moderate to high total

alkalinity (75 to 200, but not less

than 20 mg/L) and a calcium hardness of 100 to 250 mg/L CaCO3

.

Many of the principles of chemistry are abstract (e.g., carbonate-bicarbonate buffering) and difficult

to grasp. However, a fundamental

understanding of the concepts and

chemistry underlying the interactions of pH, CO2

, alkalinity and

hardness is necessary for effective

and profitable pond management.

There is no way to avoid it; water

quality is water chemistry.

References

Boyd, Claude E. 1979. Water Quality in

Warmwater Fish Ponds. Auburn University Agricultural Experiment Station.

Tucker, C.S. 1984. Carbon dioxide. in T.L.

Wellborn, Jr. and J.R. MacMillan (eds.)

For Fish Farmers 84-2. Mississippi Cooperative Extension Service.

The work reported in this publication was supported in part by the Southern Regional Aquaculture Center through Grant No. 89-38500-4516 from the United States

Department of Agriculture.