A practical understanding of water quality is essential to the fish farmer to allow assessment of environmental conditions and implementation of effective treatment strategies. Often, aquaculturists do not have extensive training in water chemistry and as a result may misinterpret or misapply information about water quality and its management. While the chemistry of water is a complex subject, most aspects of general importance to fish farmers can be simplified to allow for easier understanding and practical approaches to management.
Dissolved Oxygen
In ponds dissolved oxygen can fluctuate greatly due to Photosynthetic oxygen production by algae during the day and the continuous consumption of oxygen due to respiration. As a result of these processes, dissolved oxygen typically reaches a maximum during the late afternoon and a minimum around sunrise.
Aquaculturists know that cloudy weather, rain,
plankton die-offs and heavy stocking and feeding rates can result in low levels of dissolved oxygen which can stress or kill fish.
Oxygen is only slightly soluble in water, however, water may be frequently supersaturated with oxygen in ponds with algae blooms. As an example, at sea level at a temperature of 25 C, (77 F) pure water contains 8.11 milligrams/liter, (mg/l), of oxygen when 100% saturated, but during the afternoon hours, levels of 10 to 14 mg/l in ponds with healthy algae blooms are not uncommon. A mg/l is equivalent to one part per million in water at 4 C, (39 F). As water warms, or is raised to higher altitudes, or becomes more saline, its oxygen holding capacity declines. Water saturated with oxygen at 15 C (59 F) contains 9.76 mg/], while water at 30 c (i6 F) is saturated at 7.53 mg/I.
Most aquaculturists measure dissolved oxygen with oxygen meters or chemical test kits which give results in mg/I. Guidelines for oxygen management usually report that oxygen levels should be maintained above 4 mg/l to avoid fish stress. Most warm water fish experience significant oxygen stress at levels of 2 mg/], and that levels of less than 1 mg/l may result in fish kills. While these guidelines are accurate, fish actually respond to the percent saturation of oxygen rather than the oxygen content in water. A reading of I mg/l at 30 C (I 3.3 % saturated) is a higher concentration than I mg/l at 15 C (10.20% saturated) and hence represents more available oxygen.
Aeration is applied to sustain adequate dissolved oxygen in water. The oxygen transfer efficiency of an aerator is, in part, determined by the oxygen deficit of the water. As the oxygen content is raised towards saturation the rate of oxygen transfer by an aerator decreases. Aeration of water supersaturated with oxygen lowers the oxygen content back toward saturation. For practical purposes, emergency aeration is efficiently applied only when oxygen levels are less than 40% of saturation. Continuous low level aeration in ponds is not designed to maximize oxygen transfer efficiency but, imparts benefits due to continuous mixing of water and stabilization of oxygen fluctuations, effects which tend to minimize the potential for severe oxygen depletions and the need for emergency aeration.
Carbon Dioxide
Carbon dioxide is a minor component of the atmosphere but, is highly soluble in water. Most carbon dioxide in pond water occurs as a result of respiration and levels usually fluctuate inversely to dissolved oxygen, being low during the day and increasing at night, or whenever respiration occurs at a greater rate than photosynthesis. Carbon dioxide
Interferes with the ability of fish to extract oxygen from water and thus contributes to stress of fish during periods of low oxygen. Aerating water to improve its oxygen content drives off excess carbon dioxide. Adding quick lime (CAOH) to water rapidly removes carbon dioxide without affecting oxygen content, thereby improving the ability of fish to use the available oxygen. Carbon dioxide acts as an acid in water lowering pH as it increases in concentration. Carbonate buffers in water neutralize carbon dioxide and stabilize pH fluctuations within the range tolerated by fish. Waters low in alkalinity and hardness may experience extremes of pH due to its poor buffering against changes in carbon dioxide concentrations.
pH
The pH is a measure of the hydrogen ion concentration (H) and indicates whether the environment is acid or basic in reaction. Technically, pH is calculated by taking the negative logarithm of the activity of hydrogen ion which converts a molar concentration into a more convenient number. The pH of pure water is 7 and considered neutral, values below 7 are acidic, those above 7 basic. A pH of 7 expressed as a mole fraction indicates that 0.0000001 moles of hydrogen ion are in solution in a liter of water. A pH of 6 indicates 0.000001 ,moles of hydrogen ion are present, a tenfold increase in concentration. Thus while the pH scale gets smaller by units of one, the hydrogen ion concentration is actually increasing by a factor of 10. A pH change from pH 6 to pH 9 is actually a 1000 fold decrease in hydrogen ion concentration. Mixing equal parts of water of pH 6 and pH 9 does not result in pH 7.5, but rather pH 6.3 due to the much greater strength of the pH 6 solution. Fish can survive in waters from pH 4 to pH 11, however, a range of pH 6.5 to pH 9 is considered desirable for fish production. Extremes of pH may be directly toxic to fish or act synergistically with other dissolved ions in water such as ammonia and hydrogen sulfide to increase their toxicity.
The pH of water is greatly influenced by changes in carbon dioxide concentrations and therefore by the processes of photosynthesis and respiration. During photosynthesis carbon dioxide is removed from water and pH rises, conversely when carbon dioxide levels increase due to respiration the pH declines. Water containing sufficient levels of alkalinity and hardness derived from lime or dolomite are buffered against extreme ranges of pH. The pH of acid water ponds may be improved by liming. Ponds with excessively high pH may be treated with alum (aluminum sulfate) to rapidly, but temporarily lower pH. Treatment with gypsum (calcium sulfate) can lower pH under certain circumstances and offer longer lasting treatment, however, neither of these treatments can be used efficiently for permanent control of high pH. Fish can normally tolerate temporary extremes of high or low pH in ponds which occur as a result of photosynthesis and respiration.
Alkalinity and hardness
The term alkalinity refers to all the ions in water which T have a basic reaction. Primarily these are carbonate (CO3 2) and bicarbonate (HCO3) ions derived from limestone or dolomite and associated with calcium (Ca2) or magnesium (MG2) which comprise the principal components of the term hardness. Thus hardness and alkalinity are usually closely related terms of their concentration in water. As a means of standardizing relative concentrations of these ions they are expressed as mg/1 of CACO3 (calcium carbonate) which puts all concentrations on an equivalent basis. Waters suitable for fish production are recommended to have at least 1-0 mg/l and less than 500 mg/l of total alkalinity and hardness. Waters with these characteristics are usually fertile and have sufficient buffering capacity to avoid extremes of pH. Some species of fish are adapted to specific water types and require more narrow ranges of water quality, for spawning and larval rearing. Tropical fish breeders in hard water areas must often acidify and soften water to breed species native to certain regions of the tropics, while catfish farmers in acid or soft water areas have found it beneficial to add hardness to hatchery water to improve egg development and survival of fry. Liming is an effective procedure to increase the alkalinity and hardness of acid ponds. There is no procedure for long term acidification and softening of alkaline, hard water ponds, although water softeners and water purification systems are useful for this purpose in hatcheries.
Ammonia is a waste product of protein metabolism by fish and other pond organisms. In water, ammonia occurs either in the ionized (NH4) or un-ionized (NH3) form, dependent upon pH. Un-ionized ammonia is considerably more toxic to fish and occurs in greater proportion at high pH and warmer temperatures. As an example, at 28 C (82.4 F) and pH 8, 6.55% of the total ammonia is present in un-ionized form, however, at pH 9, 41.23% of the ammonia is un-ionized. Un-ionized ammonia is stressful to warm water fish at concentrations greater than 0. I mg/l, and lethal at concentrations approaching 0.5 mg/l. Test kits for determining ammonia in water measure total ammonia, therefore, pH should also be checked to determine if a large percentage of the ammonia is in unionized form. A pH above 8, in the presence of ammonia concentrations above 0.5 mg/I, is cause for concern. Algae use ammonia as a nitrogen source and therefore concentrations usually remain low in ponds with phytoplankton blooms. The greatest concentration of ammonia often occurs after plankton die-offs at which time pH is low due to high levels of carbon dioxide, and the majority of ammonia is present in the relatively nontoxic ionized form.
Nitrite
Nitrite (NO2) is formed as an intermediary during the biological oxidation of ammonia to nitrate, and the denitrification of nitrate to nitrogen gas. These reactions occur in soils, mud, and water. In freshwater, nitrite can increase in concentration sufficient to stress or kill fish. Nitrite is toxic because it interferes with oxygen uptake by the hemoglobin in blood and causes fish to suffocate even in the presence of oxygen. The binding of nitrite with the hemoglobin molecule gives blood a chocolate brown color, hence the condition has become known to fish farmers as brown blood disease. In medical terms it is known as methemoelobanerria. The toxicity of nitrite to fish is lessened by the presence of chlorides in water. The addition of salts, either calcium chloride or sodium chloride at rates of 20 mg/l for each 1 mg/l of nitrite has become a standard treatment for preventing nitrite poisoning in freshwater ponds. Most warm water fish can tolerate at least 0.4 mg/I of nitrite in fresh water without treatment if oxygen levels remain above 4 mg/I. Nitrite should be monitored frequently if a problem is suspected since its concentration may increase rapidly in pond water, especially during spring and fall, or when algae blooms suddenly die. Test kits for the determination of nitrite usually give results as nitrite-nitrogen (NO2 N) expressing the concentration only in terms of the nitrogen present. Toxicity data for nitrite may be reported in this form or as nitrite alone. To convert a concentration expressed as nitrite-nitrogen to nitrite multiply by 3.28.
Hydrogen sulfide
Hydrogen sulfide (HS) occurs in ponds as a result of the anaerobic decomposition of organic matter by bacteria in mud. In some areas, well water may also contain significant quantities of this gas, recognized by its characteristic rotten egg smell. Hydrogen sulfide is toxic to fish and interferes with normal respiration. Toxicity is increased at higher temperatures and a pH less than 8 when the largest percentage of hydrogen sulfide is in the toxic unionized form. Vigorous aeration or splashing is usually sufficient to remove hydrogen sulfide from well water. In ponds, hydrogen sulfide can be released from anaerobic mud when the bottom is disturbed by seining and harvest activities. Liming ponds can raise mud pH and reduce the potential for the formation of hydrogen sulfide. Potassium permangenate at 2 - 6 mg/l can be used to remove hydrogen sulfide from water and reverse the effects of its toxicity to fish.
Chlorine and chloride
Few aquaculturists are apt to mistake the difference between these two forms of elemental chloride. Chlorine (Cl2) in gaseous form or as hypochlorites are widely used for disinfection of water supplies, and in aquaculture for sterilization of equipment, tanks, and standing water in drained ponds. Chlorine acts as a powerful oxidizing agent and it is toxic to fish at concentrations of less than 0.05 mg/l. Residual chlorine in municipal water supplies is normally between 0.5 and 2.0 mg/I. Water used for fish culture should not contain any residual chlorine to be considered safe. Chlorine can be removed from water by extended periods of aeration prior to use, or more rapidly by the addition of sodium thiosulfate at a rate of 7.0 mg/l for each I mg/l of chlorine. Sodium thiosulfate is not considered toxic to fish at concentrations required to remove chlorine from water.
Chloride (C1) is a by-product of chlorine dissociation in water but is also widely associated with numerous other compounds which are highly water soluble. Chloride occurs in a range of concentrations in water and is often used as an indicator to characterize aquatic environments as saline or fresh. Fish too, are classified as freshwater or marine as determined by their physiological adaptation to salinity. Chloride is important to fish in osmoretulation and other physiological process, and is regarded as non-toxic within the tolerance range of each species. Tests to determine the chlorine or chloride content of water require completely different chemical procedures and the distinction between the two compounds and their significance in water is obvious, but occasionally confused.
