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. Carbondioxide, pH, alkalinity and hardness are interrelated and can haveprofound 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, pHindicates 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. MostpH 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 fishblood (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 theday due to respiration and photosynthesis. After sunset, dissolvedoxygen (DO) concentrations decline as photosynthesis stops and all plants and animals in the pond consume oxygen (respiration). In heavily stocked fish ponds, carbon dioxide (CO 2

) concentrations can become high as a result of respiration. The free CO 2 released duringrespiration reacts with water, producing carbonic acid (H 2 CO 3 ), and pH is lowered.

H 2O + CO 2 = H 2

CO 3 = H + + HCO 3 - Table 1 summarizes the relative changes in dissolved oxygen, CO 2 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 CO 2 levels are high and appear unaffected when CO 2 is low. Catfish can tolerate 20 to 30 mg/L CO 2 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, CO 2 rarely exceeds 5 to 10 mg/ L. High CO 2 concentrations are almost always accompanied by low dissolved oxygen concentrations (high respiration); the aerationused to increase low dissolved oxygen will, to some extent, help reduce excess CO 2 by improving itsdiffusion back into the atmosphere. Chronically high CO 2 levels can be treated chemically with hydrated lime, Ca(OH) 2. Approximately 1 mg/L of hydrated lime will remove 1 mg/L of CO 2. This treatment should not be used inwaters 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 waterswith 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 basesfound in fish ponds include carbonates, bicarbonates, hydroxides,phosphates and berates. Carbonates and bicarbonates are the mostcommon 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 orparts per million calcium carbonate (mg/L or ppm CaCO 3 ). 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 CaCO 3. Carbonate-bicarbonate alkalinity (and hardness) in surface and well waters is produced primarily through the interactions of CO 2, water and limestone. Rainwater isnaturally acidic because of exposure to atmospheric carbon dioxide. As rain falls to the earth, each droplet becomes saturated with CO 2; and pH is lowered. Wellwater is pumped from large, natural underground reservoirs (aquifers) or small, localized pockets ofunderground water (groundwater), Typically, underground water has high CO 2 concentrations, and low pH and oxygen concentrations. Carbon dioxide ishigh 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 overand percolate through soil and underground rock formations containing calcitic limestone (CaCO 3 ) or dolomitic limestone [CaMg(CO 3

) 2], the acidity produced by CO 2 will dissolve limestone and form calcium and magnesium bicarbonate salts: CaCO 3 + H 2

O + CO 2 = Ca +2 + 2HCO 3 -

or CaMg(CO 3

) 2 + 2H 2 O + 2CO 2 = Ca+2 + Mg +2 + 4HCO 3 - The resultant water has increased alkalinity, pH and hardness. 

### Alkalinity, pH and carbon dioxide concentrationsIn 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 CO 2 (i.e., carbonic acid) or other acids are required to lower pH because thereis 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, CO 2 concentrations are negligible above pH = 8.4.

2ship among alkalinity, pH and

CO 2 can be determined from Table 2. The number (factor) found in the table which corresponds to themeasured pH and water temperature is multiplied by the measured alkalinity value (mg/L as CaCO 3 ).The product of these numbers estimates CO 2 concentration (mg/L). For example, Mr. Jacobs measures a pH of 7.2, a temperature of 77°F (25°C) and total alkalinity of 103mg/L in his catfish pond. He determines the corresponding factor, 0.124, from Table 2 and multipliesthis number by the measured alkalinity, 103 mg/L. The resulting product gives him an estimate of the CO 2 concentration in his pond: 0.1 24x 103 mg/L alkalinity = 12.8 mg/L CO 2 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 CO 2 usinga 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 between6.5 and 9. In waters with low alkalinity, pH can reach dangerously low (CO 2 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. Bystabilizing 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 CO 2 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 CO 2 in photosynthesis, the pH of pond water increases as carbonic acid (i.e., CO 2) is removed. Also, phytoplankton and other plants can combine bicarbonates (HCO 3 -) to form CO 2 for photosynthesis, and carbonate (CO 3 -2 ) is released:

2HCO 3 - + phytoplankton = CO 2 (photosynthesis) + CO 3 -2 + H 2O) CO 3 -2 + H 2O = HCO 3 - + OH- (strong base) High pH could also be viewed as a decrease in hydrogen ions (H+): CO 3-2 + H + = HCO 3- or HCO 3- + H + = H 2O + CO 2 The release of carbonate converted from bicarbonate by plant life can cause pH to climb dramatically (above 9) during periods of rapidphotosynthesis 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 phytoplanktonblooms and do not commonly experience dramatic pH increases because of intense photosynthesis. 

### Hardness Water hardness is important tofish culture and is a commonly reported aspect of water quality. Itis 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. Thehardness of a water sample is reported in milligrams per liter as calcium carbonate (mg/L CaCO 3 ). Calcium carbonate hardness is a general term that indicates thetotal quantity of divalent salts present and does not specifically identify whether calcium, magnesiumand/or some other divalent salt is

causing water hardness. Hardness is commonly confusedwith alkalinity (the total concentration of base). The confusion relates to the term used to report both measures, mg/L CaCO 3. If limestone is responsible for bothhardness and alkalinity, the concentrations will be similar if notidentical. However, where sodium bicarbonate (NaHCO 3) is responsible for alkalinity it is possible to have low hardness and high alkalinity. Acidic, ground or wellwater 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 metabolicreactions). Fish can absorb calcium and magnesium directly from the water or from food.However, calcium is the most important environmental, divalentsalt in fish culture water. The presence of free (ionic), calcium in culture water helps reduce the loss ofother 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 CaCO 3hardness). 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 3 hardness) are desirable; a value of 100mg/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 3 hardness value is a reliable indication that the calcium concentration is low. However, high hardness does not necessarilyreflect 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 thathigh hardness measurements reflect high calcium levels. A CaCO 3 hardness value of 100 mg/L represents a free calcium concentration of 40 mg/L (divide CaCO 3value by 2.5) if hardness is caused by the presence of calcium only. Similarly, a CaCO 3value of100 mg/L represents a free magnesium value of 24 mg/L (divide CaCO 3value 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 3 and the difference inweights between calcium and magnesium atoms. Where hardness is caused by limestone, the CaCO 3 value usually reflects a mixture of free calcium and magnesium withcalcium 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 toraise calcium levels in soft, alkaline waters. Expense might be prohibitive if large volumes of waterneed 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 aspH increases. Higher concentrations of the toxic form of ammonia (NH 3 ) are formed in basic waters;while the less toxic form, ammonium (NH 4 +), is more prevalent inacidic waters. Since alkalinity increases pH, ammonia will be moretoxic 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 metalsbecome 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 baseswhich produce less toxic or insoluble forms of copper and zinc. High concentrations of calcium and magnesium (hardness) block the effects of copper and zinc attheir 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 lessthan 20 mg/L) and a calcium hardness of 100 to 250 mg/L CaCO 3.Many of the principles of chemistry are abstract (e.g., carbonate-bicarbonate buffering) and difficult to grasp. However, a fundamental understanding of the concepts andchemistry underlying the interactions of pH, CO 2, 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 inWarmwater 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.