### Southern Regional Aquaculture Center SRAC Publication No. 464

December 1992



# Interactions of pH, Carbon Dioxide, Alkalinity and Hardness in Fish Ponds

Water quality in fish ponds is affected bythe interactions ofseveral chemical components. Carbon dioxide, pH, alkalinity and hardness are interrelated and can have profound effects on pond productivity, the level ofstress and fish health, oxygen availability and the toxicity ofammonia as well as that ofcertain 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 overtime, usually weeks to months, depending on the pH or mineral content ofwatershed and bottom soils. 

### pH and carbon dioxide
The measure which indicates whether water is acidic orbasic is known as pH. More precisely, pH indicates the hydrogen ion concentration in water and is defined as the negative logarithm ofthe 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 0 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 William A. Wurts and Robert M. Durborow*



### blood comes into close contact with water (1- or 2-cell separation)

as it passes through the blood vessels ofthe gills and skin. A desirable range for pond water pH wouldbe close to that offish blood (i.e., 7.0 to 8.0). Fish may become stressed and die ifthe 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 (C02 ) concentrations can become high as a result ofrespiration. The free C02 released during respiration reacts with water, producing carbonic acid (H2 COJ, and pH is lowered.

HzO + 

### C02

### =H2C03

### =H+ + 

### HC03- Table 1summarizes the relative changes in dissolved oxygen, C02 and pH over 24 hours. Carbon dioxide rarely causes direct toxicity to fish. However, high concentrations lower pond pH and limit the capacity offish 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 permillion, ppm), fish may suffocate when C02levels are high and appear unaffected when C02 is 

### low. Catfish can tolerate 20 to 30 mg/L C02 ifaccumulation is slow and dissolved

Death

### ~ 

# I Desired range for fish production 

# I I I Death**~**

u 4.0 6.5 7 9.0 11 14

Figure 1. pH scale showing recommended range.I

-- - I Table 1. Relative concentration changes for dissolved oxygen, carbon dioxide and pH in ponds over 24 hours. cause ofbacterial 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 (CaCOJ

or dolomitic limestone [CaMg(COJ 2], the acidity produced by C02will dissolve limeDissolved Time Oxygen Daylight Increases Nightime Decreases
Tucker (1984).

oxygen concentrations are above 5 mg/L. In a reservoir or natural pond, C02rarely exceeds 5 to 10 mg/L. High C02concentrations 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 C02by improving its diffusion back into the atmosphere. Chronically high C02levels can be treated chemically with hydrated lime, Ca(OH) 2• Approximately 1mg/L ofhydrated lime will remove 1 mg/L ofC0 2• 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 ofbase 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 ofacid (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 CaC03). A total alkalinity of 20 mg/L or more is necessary for good pond productivity. A desirable range of Change
Carbon Dioxide pH

Decreases Increases

- Increases Decreases

total alkalinity for fish culture is between 75 and 200 mg/L CaC0 3• Carbonate-bicarbonate alkalinity (and hardness) in surface and well waters is produced primarily through the interactions ofC02, 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 C0 2; and pH is lowered. Well wateris pumped from large, natural underground reservoirs (aquifers) or small, localized pockets of underground water (groundwater), Typically, underground water has high C02concentrations, and low pH and oxygen concentrations. Carbon dioxide is high in underground water bestone and form calcium and magnesium bicarbonate salts: CaC03+ H 20 + C02

### = Ca+2+ 2HC03-
or CaMg(COJ 2+ 2H2 0 + 2C02= Ca+2 + Mg+2+ 4HCosThe resultant water has increased alkalinity, pH and hardness. 

### Alkalinity, pH and carbon dioxide concentrations
In waterwith 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 C02 (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)

## ~ 5 10 15 20 25 30 35 6.0 2.915 2.539 2.315 2,112 1.970 1.882 1.839 6.2 1.839 1.602 1.460 1.333 1.244 1.187 1.160 6.4 1.160 1.010 0.921 0.841 0.784 0.749 0.732 6.6 0.732 0.637 0.582 0.531 0.495 0.473 0.462 6.8 0.462 0.402 0.367 0.335 0.313 0.298 0.291 7.0 0.291 0.254 0.232 0.211 0.197 0.188 0.184 7.2 0.184 0.160 0.146 0.133 0.124 0.119 0.116 7.4 0.116 0.101 0.092 0.084 0.078 0.075 0.073 7.6 0.073 0.064 0.058 0.053 0.050 0.047 0.046 7.8 0.046 0.040 0.037 0.034 0.031 0.030 0.030 8.0 0.029 0.025 0.023 0.021 0.020 0.019 0.018 8.2 0.018 0.016 0.015 0.013 0.012 0.012 0.011 8.4 0.012 0.010 0.009 0.008 0.008 0.008 0.007 Tucker (1984). "Factors should be multiplied by total alkalinity (mg/L) to get carbon dioxide (mg/L). For**~ractical ~ur~oses,**  C0 2concentrations are negligible above**~H** 

## =8.4.

2ship among alkalinity, pH and C02can be determined from Table 2. The number (factor) found in the table which corresponds to the measured pH and water temperature is multiplied bythe measured alkalinity value (mg/L as CaCOJ. The product ofthese numbers estimates C02concentration (mg/L).
For example, Mr. Jacobs measures a pH of7.2, a temperature of7rF (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 numberbythe measured alkalinity, 103 mg/L. The resulting product gives him an estimate of the C02concentration in his pond: 0.1 24x 103 mg/L alkalinity= 12.8 mg/L C02 A promptpH measurement within 30 minutes ofwater sampling is required to minimize

error when using this method. Due to several sources oferror that can occur with this method, direct measurement of C02using

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 ofwell buffered water normally fluctuates between 6.5 and 9. In waters with low alkalinity, pH can reach dangerously low (C02and 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 C02and increase the pH**11nr-----------------=~~~-----------------**

9

7

6

5**~=~~~l(···<** Low Alkalinity Water

- High Alkalinity Water

Early Morning Mid Afternoon Early 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 C0 2in photosynthesis, the pH ofpond water increases as carbonic acid (i.e., C02 ) is removed. Also, phytoplankton and otherplants can combine bicarbonates (HC03) to form C02for photosynthesis, and carbonate (CO/) is released:
2HC0 3-+ phytoplankton = C02 (photosynthesis) + CO/+ H2 0) CO/+ H20 

## = HC03-+ OH- ( strong base) High pH could also be viewed as a decrease in hydrogen ions (H+): CO-2 + H+= HCO 

### -or HC0 3

3-+ H+= H20 +3 C02 The release ofcarbonate converted from bicarbonate by plant life can
cause pH to climb dramatically (above 9) during periods ofrapid photosynthesis by dense phytoplankton (algal) blooms. This rise in pH can occur in low alkalinity water (20 to 50 mg/L) orin 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. Ifcalcium, 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 ofintense photosynthesis. 

### Hardness
Water hardness is important to fish culture and is a commonlyreported aspect ofwater quality. It is a measure ofthe quantity ofdivalent ions (for this discussion, salts with two positive charges) such as calcium, magnesium and/oriron in water. Hardness

can be a mixture ofdivalent salts; however, calcium and magnesium

are the most common sources of waterhardness. Hardness is traditionally measured by chemical titration. The hardness ofa water sample is reported in milligrams perliter as calcium carbonate (mg/L CaCOJ. Calcium carbonate hardness is a general termthat indicates the total quantity ofdivalent salts present and does not specifically identify whether calcium, magnesium

### and/ or some other divalent salt Is 

### causing waterhardness. Hardness Is 

### commonly confused with alkalinity (the total concentration ofbase). The confusion relates to the termusedto report both measures, mg/L CaCO,. If 

### limestone Is 

### responsible for both hardness and alkalinity, the concentrations will be similarifnot identical. However, where sodium bicarbonate (NaHCOJ is responsible for alkalinity itis possible to have low hardness and high alkalinity. Acidic, ground or well water can have low or high hard

### ness and has

### little orno alkalinity. Calcium andmagnesium are essential in the biological processes offish (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 Im

### portant environmental, divalent salt In fish culture water. The presence offree (ionic), calcium In culture waterhelps reduce the loss of other salts (e.g., sodium and potassium) from fish bodyfluids (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 shownthat environmental calcium Is 

### also required to re-absorb these lost salts. In low calcium water, fish canlose Oeak) substantial quantities ofsodium and potassiumInto the water. Body energy Is 

### used to re-absorb the lost salts. For some species (e.g., reddrum and stripedbass), 

### relatively high concentrations of calcium hardness are required for survival. A recommendedrange for free calcium In culture waters Is 

### 25 to 100 mg/L (63 to 250 mg/L CaCO, hardness). Channel catfish can tolerate low calcium concentrations

as long as their feed contains a minimum level ofmineral calcium but may grow slowly underthese conditions. Similarly, rainbow trout can tolerate waters withfree calcium concentrations as low as 10 mg/L if

### pH is above 6.5. If freshwater culture ofstripedbass, reddrum orcrawfish Is 

### being considered, free calcium concentrations In the 40 to 100 mg/Lrange (100 to 250 mg/L as CaCO,hardness) are desirable; a value of I00 mg/L (250 mg/ L calcium hardness) matches the calcium concentration offish blood. Tests specific for calcium hardness shouldbe performed on samples ofthe water source being considered for these animals. A low CaCO,hardness value Is 

### a reliable indication thatthe calcium concentration is low. However, 

### high hardness does notnecessarily reflect a high calcium concentration. But, since limestone Is 

### commonIn the soil andbedrock ofthe southern United States, itwould be reasonably safe to assume that highhardness measurementsreflect high calcium levels. A CaCO,hardness value of 100 mg/Lrepresents a free calcium concentration of40 mg/L (divide CaCO,value by 2.5) if

### hardness is caused by the presence ofcalcium only. Similarly, a CaCO,value of 100 mg/L represents a free magnesium value of24 mjl/L (divide CaCO,value by 4.12) ifhardness is causedbymagnesium only. These factors (2.5 and 4.12) are related to the molecular weight of CaCO,and the difference In weights between calcium andmagnesium atoms. Where hardness Is 

### caused by limestone, the CaCO, valueusuallyreflects a mixture of free calcium andmagnesiumwith calcium being the predominantdivalent 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 of8.3 or greater, agricultural limestone will not dissolve. Agricultural gypsum (calcium sulfate) or food grade calcium chloride couldbe used to raise calcium levels in soft, alka

### line waters. Expense might be prohibitive iflargevolumes ofwater need treatroent Identifying a suitable water source maybe more practical. 

### Effects of pH, alkalinity and hardness on ammonia and metal toxicities
Ammoniabecomes more toxic as pH Increases. Higher concentrations ofthe toxic form ofammonia (NHJ are formed In basic waters; while the less toxic form, ammonium (NH;), is more prevalent In acidic waters. Since alkalinity Increases pH, ammonia will be more toxic In waters withhigh total alkalinity. Hardness Is 

### not typically associated with ammonia toxicity. Metals such as copper andzinc are commonly used around aquatic environments (tanks, 

### plumbing and copper sulfate). These metals become more soluble in acidic environments. The soluble orfree Ionic forms ofthese metals are toxic to fish. Hightotal alkalinity increases pH and available bases which produce less toxic orInsoluble forms ofcopper andzinc. High concentrations ofcalcium and magnesium (hardness) block the effects ofcopper and zinc at their sites oftoxic action. Therefore, copper andzinc are more toxic to fJSh in soft, acidic waters withlow total alkalinity. Ideally, an aquaculture pond should have a pHbetween 6.5 and 9 as well as moderate to high total alkalinity (75 to 200, butnotless than 20 mg/L) and a calcium hardness of 100 to 250 mg/L CaCO,. Many ofthe principles ofchemistry are abstract (e.g., carbonate-bicarbonate buffering) and difficult to grasp. However, a fundamental understanding ofthe concepts and chemistry underlying the interactions ofpH, CO,. alkalinity and hardness Is 

### necessary for effective and profitable pond management. There is no wayto 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. inT.L. Wellborn, Jr. aod J.R. MacMlllao (eels.) For Fish Fanners 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.