SRAC Publication No. 464

IISouthernRegionalAquacultureCenter

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 af-fected by the interactions of sev-eral chemical components. Carbondioxide, pH, alkalinity and hard-ness are interrelated and can haveprofound effects on pond produc-tivity, the level of stress and fishhealth, oxygen availability and thetoxicity of ammonia as well as thatof certain metals. Most features ofwater quality are not constant.Carbon dioxide and pH concentra-tions fluctuate or cycle daily. Alka-linity and hardness are relativelystable but can change over time,usually weeks to months, depend-ing on the pH or mineral contentof watershed and bottom soils.

pH and carbon dioxideThe measure which indicateswhether water is acidic or basic isknown as pH. More precisely, pHindicates the hydrogen ion concen-tration in water and is defined asthe negative logarithm of themolar hydrogen ion concentration(-log [H+]). Water is consideredacidic when pH is below 7 andbasic when pH is above 7. MostpH values encountered fall be-tween O and 14. The recom-mended pH range for aquacultureis 6.5 to 9.0 (Figure 1).

Fish and other vertebrates have anaverage blood pH of 7.4. Fish

* Kentucky State University

blood comes into close contactwith water (1- or 2-cell separation)as it passes through the blood ves-sels of the gills and skin. A desir-able range for pond water pHwould be close to that of fishblood (i.e., 7.0 to 8.0). Fish may be-come stressed and die if the pHdrops below 5 (e.g., acidic runoff)or rises above 10 (e.g., low alkalin-ity combined with intense photo-synthesis by dense algal blooms –phytoplankton or filamentousalgae).

Pond pH varies throughout theday due to respiration and photo-synthesis. After sunset, dissolvedoxygen (DO) concentrations de-cline as photosynthesis stops andall plants and animals in the pondconsume oxygen (respiration). Inheavily stocked fish ponds, carbondioxide (CO2) concentrations can

become high as a result of respira-tion. The free CO2 released duringrespiration reacts with water, pro-ducing carbonic acid (H2CO3), andpH is lowered.

H2O + CO2 = H2CO3 = H+ + HCO3

-

Table 1 summarizes the relativechanges in dissolved oxygen, CO2

and pH over 24 hours.

Carbon dioxide rarely causes di-rect toxicity to fish. However,high concentrations lower pondpH and limit the capacity of fishblood to carry oxygen by loweringblood pH at the gills. At a givendissolved oxygen concentration(e.g., 2 mg/L, milligrams per liter;same as parts per million, ppm),fish may suffocate when CO2 lev-els are high and appear unaffectedwhen CO2 is low. Catfish can tol-erate 20 to 30 mg/L CO2 if accu-mulation 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 CarbonTime Oxygen Dioxide pH

Daylight Increases Decreases Increases

Nightime Decreases Increases Decreases

Tucker (1984).

oxygen concentrations are above 5mg/L. In a reservoir or naturalpond, CO2 rarely exceeds 5 to 10mg/ L.

High CO2 concentrations are al-most always accompanied by lowdissolved oxygen concentrations(high respiration); the aerationused to increase low dissolved oxy-gen will, to some extent, help re-duce excess CO2 by improving itsdiffusion back into the atmos-phere. Chronically high CO2 lev-els can be treated chemically withhydrated lime, Ca(OH)2. Approxi-mately 1 mg/L of hydrated limewill remove 1 mg/L of CO2. Thistreatment should not be used inwaters with poor buffering capac-ity (low alkalinity) because pHcould rise to levels lethal to fish.Also, fish could be endangered ifhydrated lime is added to waterswith high ammonia concentra-tions. High pH increases the toxic-ity of ammonia.

AlkalinityThe quantity of base present inwater defines what is known astotal alkalinity. Common basesfound in fish ponds include car-bonates, bicarbonates, hydroxides,phosphates and berates. Carbon-ates and bicarbonates are the mostcommon and most important com-ponents of alkalinity. Alkalinity ismeasured by the amount of acid(hydrogen ion) water can absorb(buffer) before achieving a desig-nated pH. Total alkalinity is ex-pressed as milligrams per liter orparts per million calcium carbon-ate (mg/L or ppm CaCO3). Atotal alkalinity of 20 mg/L ormore is necessary for good pondproductivity. A desirable range of

total alkalinity for fish culture isbetween 75 and 200 mg/L CaCO3.

Carbonate-bicarbonate alkalinity(and hardness) in surface and wellwaters is produced primarilythrough the interactions of CO2,water and limestone. Rainwater isnaturally acidic because of expo-sure to atmospheric carbon diox-ide. As rain falls to the earth, eachdroplet becomes saturated withCO2; and pH is lowered. Wellwater is pumped from large, natu-ral underground reservoirs (aqui-fers) or small, localized pockets ofunderground water (groundwa-ter), Typically, undergroundwater has high CO2 concentra-tions, and low pH and oxygen con-centrations. Carbon dioxide ishigh in underground water be-

cause of bacterial processes in thesoils and various underground,particulate mineral formationsthrough which water moves. Asground- or rainwaters flow overand percolate through soil and un-derground rock formations con-taining calcitic limestone (CaCO3)or dolomitic limestone[CaMg(CO 3)2], the acidity pro-duced by CO2 will dissolve lime-stone and form calcium and mag-nesium bicarbonate salts:

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

-

orCaMg(CO 3)2 + 2H2O + 2CO2 =

Ca+2 + Mg+2 + 4HCO 3

-

The resultant water has increasedalkalinity, pH and hardness.

Alkalinity, pH and carbondioxide concentrationsIn water with moderate to high al-kalinity (good buffering capacity)and similar hardness levels, pH isneutral or slightly basic (7.0 to 8.3)and does not fluctuate widely.Higher amounts of CO2 (i.e., car-bonic acid) or other acids are re-quired to lower pH because thereis more base available to neutral-ize or buffer the acid. The relation-

Table 2. Factors for calculating carbon dioxide concentrationsin water with known pH, temperature and alkalinitymeasurements. a

Temperatures (°C)I

pH 5 10 15

6.0 2.9156.2 1.8396.4 1.1606.6 0.7326.8 0.4627.0 0.2917.2 0.1847.4 0.1167.6 0.0737.8 0.0468.0 0.0298.2 0.0188.4 0.012

2.5391.6021.0100.6370.4020.2540.1600.1010.0640.0400.0250.0160.010

2.3151.4600.9210.5820.3670.2320.1460.0920.0580.0370.0230.0150.009

20

2,1121.3330.8410.5310.3350.2110.1330.0840.0530.0340.0210.0130.008

25 30 35

1.9701.2440.7840.4950.3130.1970.1240.0780.0500.0310.0200.0120.008

1.8821.1870.7490.4730.2980.1880.1190.0750.0470.0300.0190.0120.008

1.8391.1600.7320.4620.2910.1840.1160.0730.0460.0300.0180.0110.007

Tucker (1984).aFactors 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 andCO2 can be determined from Table2. The number (factor) found inthe table which corresponds to themeasured pH and water tempera-ture is multiplied by the measuredalkalinity value (mg/L as CaCO3).The product of these numbers esti-mates CO2 concentration (mg/L).

For example, Mr. Jacobs measuresa pH of 7.2, a temperature of 77°F(25°C) and total alkalinity of 103mg/L in his catfish pond. He de-termines the corresponding factor,0.124, from Table 2 and multipliesthis number by the measured alka-linity, 103 mg/L. The resultingproduct gives him an estimate ofthe CO2 concentration in his pond:

0.1 24x 103 mg/L alkalinity =12.8 mg/L CO2

A prompt pH measurementwithin 30 minutes of watersampling is required to minimizeerror when using this method.Due to several sources of errorthat can occur with this method,direct measurement of CO2 usinga chemical test procedure is pre-ferred,

Alkalinity, pH andphotosynthesisThe bases associated with alkalin-ity react with and neutralize acids.Carbonates and bicarbonates canreact with both acids and basesand buffer (minimize) pHchanges. The pH of well bufferedwater normally fluctuates between6.5 and 9. In waters with low alka-linity, pH can reach dangerouslylow (CO2 and carbonic acid fromhigh respiration) or dangerouslyhigh (rapid photosynthesis) levels(Figure 2).

Phytoplankton are microscopic ornear microscopic, aquatic plantswhich are responsible for most ofthe oxygen (photosynthesis) andprimary productivity in ponds. Bystabilizing pH at or above 6.5, alka-linity improves phytoplanktonproductivity (pond fertility) byincreasing nutrient availability(soluble phosphate concentra-tions). Alkalinities at or above 20mg/L trap CO2 and increase the

pH11

10

9

8

7

6

Early Mid EarlyMorning Afternoon Morning

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

concentrations available for photo-synthesis.

Because phytoplankton use CO2 inphotosynthesis, the pH of pondwater increases as carbonic acid(i.e., CO2) is removed. Also, phyto-plankton and other plants cancombine bicarbonates (HCO3

-) toform CO2 for photosynthesis, andcarbonate (CO3

-2) is released:

2HCO3

- + phytoplankton =CO2 (photosynthesis) + CO3

-2 + H2O)

CO 3

-2 + H2O = HCO3

- + OH-(strong base)

High pH could also be viewed as adecrease in hydrogen ions (H+):

CO3

-2 + H+ = HCO3

- orHCO3

- + H+ = H2O + CO2

The release of carbonate convertedfrom bicarbonate by plant life cancause pH to climb dramatically(above 9) during periods of rapidphotosynthesis by dense phyto-plankton (algal) blooms. This risein pH can occur in low alkalinitywater (20 to 50 mg/L) or in waterwith moderate to high bicarbonatealkalinity (75 to 200 mg/L) thathas less than 25 mg/L hardness.High bicarbonate alkalinity in softwater is produced by sodium andpotassium carbonates which aremore soluble than the calcium andmagnesium carbonates that cause

3

hardness. If calcium, magnesiumand photosynthetically producedcarbonate are present when pH isgreater than 8.3, limestone isformed. Ponds with alkalinitiesbelow 20 mg/L do not usuallysupport good phytoplanktonblooms and do not commonly ex-perience dramatic pH increases be-cause of intense photosynthesis.

HardnessWater hardness is important tofish culture and is a commonly re-ported aspect of water quality. Itis a measure of the quantity of di-valent ions (for this discussion,salts with two positive charges)such as calcium, magnesiumand/or iron in water. Hardnesscan be a mixture of divalent salts;however, calcium and magnesiumare the most common sources ofwater hardness.

Hardness is traditionally meas-ured by chemical titration. Thehardness of a water sample is re-ported in milligrams per liter ascalcium carbonate (mg/L CaCO3).Calcium carbonate hardness is ageneral term that indicates thetotal quantity of divalent salts pre-sent and does not specifically iden-tify whether calcium, magnesium

and/or some other divalent salt iscausing water hardness.

Hardness is commonly confusedwith alkalinity (the total concentra-tion of base). The confusion re-lates to the term used to reportboth measures, mg/L CaCO3. Iflimestone is responsible for bothhardness and alkalinity, the con-centrations will be similar if notidentical. However, where so-dium bicarbonate (NaHCO3) is re-sponsible for alkalinity it is possi-ble to have low hardness and highalkalinity. Acidic, ground or wellwater can have low or high hard-ness and has little or no alkalinity.

Calcium and magnesium are es-sential in the biological processesof fish (bone and scale formation,blood clotting and other metabolicreactions). Fish can absorb cal-cium and magnesium directlyfrom the water or from food.However, calcium is the most im-portant environmental, divalentsalt in fish culture water. The pres-ence of free (ionic), calcium in cul-ture water helps reduce the loss ofother salts (e.g., sodium and potas-sium) from fish body fluids (i.e.,blood). Sodium and potassiumare the most important salts in fishblood and are critical for normalheart, nerve and muscle function.Research has shown that environ-mental calcium is also required tore-absorb these lost salts. In lowcalcium water, fish can lose (leak)substantial quantities of sodiumand potassium into the water.Body energy is used to re-absorbthe lost salts. For some species(e.g., red drum and striped bass),relatively high concentrations ofcalcium hardness are required forsurvival.

A recommended range for free cal-cium in culture waters is 25 to 100mg/L (63 to 250 mg/L CaCO3

hardness). Channel catfish can tol-erate low calcium concentrationsas long as their feed contains aminimum level of mineral calciumbut may grow slowly under theseconditions. Similarly, rainbowtrout can tolerate waters with freecalcium concentrations as low as10 mg/L if pH is above 6.5. If

freshwater culture of striped bass,red drum or crawfish is being con-sidered, free calcium concentra-tions in the 40 to 100 mg/L range(100 to 250 mg/L as CaCO3 hard-ness) are desirable; a value of 100mg/L (250 mg/ L calcium hard-ness) matches the calcium concen-tration of fish blood. Tests specificfor calcium hardness should beperformed on samples of thewater source being considered forthese animals.

A low CaCO3 hardness value is areliable indication that the calciumconcentration is low. However,high hardness does not necessarilyreflect a high calcium concentra-tion. But, since limestone is com-mon in the soil and bedrock of thesouthern United States, it wouldbe reasonably safe to assume thathigh hardness measurements re-flect high calcium levels.

A CaCO3 hardness value of 100mg/L represents a free calciumconcentration of 40 mg/L (divideCaCO3 value by 2.5) if hardness iscaused by the presence of calciumonly. Similarly, a CaCO3 value of100 mg/L represents a free magne-sium value of 24 mg/L (divideCaCO3 value by 4.12) if hardnessis caused by magnesium only.These factors (2.5 and 4.12) are re-lated to the molecular weight ofCaCO3 and the difference inweights between calcium and mag-nesium atoms. Where hardness iscaused by limestone, the CaCO3

value usually reflects a mixture offree calcium and magnesium withcalcium being the predominant di-valent salt.

Agricultural limestone can be usedto increase calcium concentrations(and carbonate-bicarbonate alka-linity) in areas with acid waters orsoils. However, at a pH of 8.3 orgreater, agricultural limestone willnot dissolve. Agricultural gypsum(calcium sulfate) or food grade cal-cium chloride could be used toraise calcium levels in soft, alka-line waters. Expense might be pro-hibitive if large volumes of waterneed treatment. Identifying a suit-able water source may be morepractical.

Effects of pH, alkalinityand hardness on ammoniaand metal toxicitiesAmmonia becomes more toxic aspH increases. Higher concentra-tions of the toxic form of ammonia(NH3) are formed in basic waters;while the less toxic form, ammo-nium (NH4

+), is more prevalent inacidic waters. Since alkalinity in-creases pH, ammonia will be moretoxic in waters with high total alka-linity. Hardness is not typicallyassociated with ammonia toxicity.

Metals such as copper and zinc arecommonly used around aquaticenvironments (tanks, plumbingand copper sulfate). These metalsbecome more soluble in acidic en-vironments. The soluble or freeionic forms of these metals aretoxic to fish. High total alkalinityincreases pH and available baseswhich produce less toxic or insol-uble forms of copper and zinc.High concentrations of calciumand magnesium (hardness) blockthe effects of copper and zinc attheir sites of toxic action. There-fore, copper and zinc are moretoxic to fish in soft, acidic waterswith low total alkalinity.

Ideally, an aquaculture pondshould have a pH between 6.5 and9 as well as moderate to high totalalkalinity (75 to 200, but not lessthan 20 mg/L) and a calcium hard-ness of 100 to 250 mg/L CaCO3.Many of the principles of chemis-try are abstract (e.g., carbonate-bi-carbonate buffering) and difficultto grasp. However, a fundamentalunderstanding of the concepts andchemistry underlying the interac-tions of pH, CO2, alkalinity andhardness is necessary for effectiveand profitable pond management.There is no way to avoid it; waterquality is water chemistry.

ReferencesBoyd, Claude E. 1979. Water Quality in

Warmwater Fish Ponds. Auburn Uni-versity 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 Co-operative 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 StatesDepartment of Agriculture.