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 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 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 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 (C02) concentrations can 

become high as a result of respiration. The free C02 released during 

respiration reacts with water, producing carbonic acid (H2COJ, and 

pH is lowered. 

HzO + C02 = H2C03 = H+ + HC03-

Table 1 summarizes 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 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 C02 levels are high and appear unaffected 

when C02 is low. Catfish can tolerate 20 to 30 mg/L C02 if accumulation 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 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 (CaCOJ 

or dolomitic limestone 

[CaMg(COJ 2], the acidity produced by C02 will dissolve limeDissolved 

Time Oxygen 

Daylight Increases 

Nightime Decreases 

Tucker (1984). 

oxygen concentrations are above 5 

mg/L. In a reservoir or natural 

pond, C02 rarely exceeds 5 to 10 

mg/L. 

High C02 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 C02 by improving its 

diffusion back into the atmosphere. Chronically high C02 levels can be treated chemically with 

hydrated lime, Ca(OH) 2• Approximately 1 mg/L of hydrated lime 

will remove 1 mg/L of C02• 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 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 CaC03• 

Carbonate-bicarbonate alkalinity 

(and hardness) in surface and well 

waters is produced primarily 

through the interactions of C02, 

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 

C02; 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 C02 concentrations, and low pH and oxygen concentrations. Carbon dioxide is 

high in underground water bestone and form calcium and magnesium bicarbonate salts: 

CaC03+ H20 + C02= Ca+2+ 2HC03

-

or 

CaMg(COJ 2+ 2H20 + 2C02= 

Ca+2+ Mg+2+ 4HCosThe 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 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, C02 concentrations are negligible above ~H = 8.4. 

2 
ship among alkalinity, pH and 

C02 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 CaCOJ. 

The product of these numbers estimates C02 concentration (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 number by the measured alkalinity, 103 mg/L. The resulting 

product gives him an estimate of 

the C02 concentration in his pond: 

0.1 24x 103 mg/L alkalinity= 

12.8 mg/L C02 

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 C02 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 (C02 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 C02 and 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 C02 in 

photosynthesis, the pH of pond 

water increases as carbonic acid 

(i.e., C02

) is removed. Also, phytoplankton and other plants can 

combine bicarbonates (HC03) to 

form C02 for photosynthesis, and 

carbonate (CO/) is released: 

2HC03-+ phytoplankton = 

C02(photosynthesis) + CO/+ H20) 

CO/+ H20 = HC03-+ OH-

( strong base) 

High pH could also be viewed as a 

decrease in hydrogen ions (H+): 

CO-2 + H+= HCO -or 

HC03

3

-+ H+= H20 +

3

C02 

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 CaCOJ. 

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 CaCO,. If 

limestone Is responsible for both 

hardness and alkalinity, the concentrations will be similar if not 

identical. However, where sodium bicarbonate (NaHCOJ 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 Oeak) 

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 CaCO, 

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 CaCO,hardness) are desirable; a value of I 00 

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 CaCO,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 CaCO,hardness value of 100 

mg/L represents a free calcium 

concentration of 40 mg/L (divide 

CaCO, value by 2.5) if hardness is 

caused by the presence of calcium 

only. Similarly, a CaCO, value of 

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

CaCO, 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 

CaCO,and the difference In 

weights between calcium and magnesium atoms. Where hardness Is 

caused by limestone, the CaCO, 

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 treatroent 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 

(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 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 fJSh 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 CaCO,. 

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, CO,. 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. 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.