Water quality and Water quality Management in Aquaculture

Aquaculture can be defined as the high-density production of fish, shellfish and plant forms in a controlled environment. Stocking rates for high-density aquaculture are typically thousand fold greater than wild environments. Modern fish culturists employ both open and close systems to raise fish. Open systems, such as, the raceways (used in hatcheries of both finfish and shellfish and also in eel, trout culture) are characterized by rapid turnover of water. Closed systems are commonplace in pond culture of carps, catfishes, tilapia, sea bass, prawn and shrimp among others. Closed aquaculture systems do not have rapid turnover of water, but do not have a high surface to volume ratio facilitating exchange of gases, nutrients, energy etc. with the surroundings. Such closed system, intensified, high-density aquaculture forms the basis of concern.

The different forms of high density, intensive aquaculture is quite similar because they all obey the same set of physical and chemical principles. These principles compose the subject of water chemistry and its net result i.e. the water quality. Poor water chemistry leads to deteriorate water quality, which causes stress to the organisms being raised. Efficient feed conversion, growth and marketability of the final product cannot occur unless the pond system is balanced or in harmony with nature. Therefore the overriding concern of the fish culturist is to maintain, ‘balance’ or ‘equilibrium conditions’ with respect to water chemistry and its natural consequence, good water quality.

Water quality for aquaculturists refers to the quality of water that enables successful propagation of the desired organisms. The required water quality is determined by the specific organisms to be cultured and has many components that are interwoven. Sometimes a component can be dealt with separately, but because of the complex interaction between components, the composition of the total array must be addressed. Growth and survival, which together determine the ultimate yield, are influenced by a number of ecological parameters and managerial practices. High stocking density of fish or crustaceans in ponds usually exacerbates problems with water quality and sediment deterioration.

Wastes generated by aquaculture activity (faeces and unconsumed feed) first settle in the bottom, as a consequence of organic waste and metabolites of degraded organic matter is accumulated in sediment and water. Part of the waste is flushed out of the ponds immediately or late after the organic matter has been degraded.

Culture of penaeids has become intensified since 1986. In an intensive system of Penaeus monodon culture salinity, pH and dissolved oxygen were the main parameters, which demonstrated fluctuations of 19.5-27.5 ‰, 7.4-8.2 and 4.66-8.25 mg/l respectively. In addition, ammonia-N (un-ionized plus ionized ammonia as nitrogen) increased exponentially with culture period, and jumped to 6.5 mg/l after 75 days of cultivation.

Low dissolved oxygen level is the major limiting water quality parameter in aquaculture systems. A critically low dissolved oxygen level occurs in ponds particularly when algal blooms die-off and subsequent decomposition of algal blooms and can cause stress or mortality of prawns in ponds. Chronically low dissolved oxygen levels can reduce growth, feeding and moulting frequency.

Another major consequence of aquaculture production is a high degree of variability in the concentration of dissolved nitrates, nitrites and ammonia. High feeding rates observed in prawn farms lead to eutrophic conditions characterised by substantial phytoplankton blooms. These blooms ultimately senesce and cause rapid increasing in ammonia levels in the ponds. The environmental conditions that create high ammonia concentrations may also cause increases in nitrite concentration. Both ammonia and nitrite can be directly toxic to culture organisms or can induce to sublethal stress in culture populations that results in lowered resistance to diseases.

Ammonia accumulates in culture systems following microbial decomposition of organic material and with some fertilization practices. Microbial decomposition leads to low oxygen concentrations. Low dissolved oxygen concentration increases the toxicity of ammonia to culture organisms. In an aqueous ammonia solution unionized ammonia exists in equilibrium with ionized ammonia and hydroxide ions. The unionized form is usually toxic as it has high lipid solubility and it is able to diffuse quite readily across the cell membrane. Ammonia is utilized as energy source by nitrifying bacteria (Nitrasomonas and Nitrobactor) and oxidised it to nitrite and nitrate.

The production of brackishwater shrimps, which showed a strong increase in the 70's and 80's, showed a declining trend during 90’s in most parts of the world. In general the decrease in shrimp production of aquaculture is attributed to over intensification, leading to the deterioration of the surrounding environment and of pond water and pond sediment quality. Stress reduced shrimps' resistance to pathogenic diseases, resulting in mass mortality. Development of aquaculture activities at a particular site cannot be carried out only by considering planned facilities and the quality of water on the site at its origin but also on the aspects of water quality management.

There is a strong relationship between the quality of the water in the pond and that in the water-surrounding environment. Degradation of surrounding water quality will be faster unless proper water quality management techniques are not implemented in the ever-increasing aquaculture system.

Aquaculture pond dynamics

Aquaculture ponds are a living dynamic systems they exhibits continuous and constant fluctuations. The pond undergoes a vast collection of both chemical reactions and physical changes. Exchange of atmospheric gases including Oxygen (O2), nitrogen (N2) and Carbon dioxide (CO2) with the pond water are vital to the process of fish metabolism and plant photosynthesis. Inorganic substances (minerals) dissolve from the pond walls and bottom while precipitation of dissolved minerals occurs. Physicals exchanges between the pond its surroundings include absorption of sunlight (radiant energy) to fuel photosynthesis and supply oxygen with in the pond, heat exchange and volume changes caused by evaporation and precipitation (rain). Changes in the volume of a pond are very important as they affect the concentration of dissolved substances and correspondingly requirements for treatment. Hence, the pond dynamics not only depend on its own characters and conditions but also on the surrounding atmospheric weather conditions. Good production from aquaculture ponds can be achieved when the pond and surroundings make chemical and physical exchanges at a steady state. When all of the processes balance, a state of equilibrium is achieved. Pond equilibrium is the optimum set of conditions for aquaculture, a state completely in harmony with nature.

Water chemistry

A guiding principle of aquaculture is that water quality and hence efficient production are a direct consequence of good water chemistry. Water may be considered as a ‘binder’ or ‘matrix’ in which the dissolved gases, inorganic substances (minerals), as well as organic matter prevails. In addition to dissolved substance, the water matrix gives support to microorganisms, plant and animal life forms and provides a medium for chemical exchange among these populations. However, water is itself relatively chemically inert, physically water has a high heat capacity (holds heat efficiently), is relatively ‘polar’ affording it the ability to act as an excellent solvent and is also quite dense. Its boiling point is quite high compared to similar molecules and its freezing point quite low. Therefore, water exists as a liquid over a rather broad range of temperature making it a most suitable medium for the support of life forms.

The maintenance of good water quality is essential for both survival and optimum growth of culture organisms. The levels of metabolites in pond water that can have an adverse effect on growth are generally an order of magnitude lower than those tolerated by fishes/prawns/shrimps for survival. Good water quality is characterized by adequate oxygen and limited levels of metabolites. The culture organisms, algae and microorganisms such as bacteria produce metabolites in a pond. The major source of nutrients in aquaculture is the feed. Because large quantities of feed are loaded in ponds, excess feed, fecal matter and other metabolites become available in large quantities for the growth of algae and microorganisms.

At one point, the increase in population of algae and microorganisms is exponential. This usually occurs during the second half of the culture period because of available nutrients. About 30% of the total feed consumption is loaded into the pond during the third quarter of the culture period and about 50% is loaded during the last quarter. The algae and microbial population increases until a factor required for growth becomes limiting, after which a sudden decrease in the population can occur. This is referred to as a “collapse” or a “die-off”. The sudden increase and decrease in algal and microbial population can cause drastic changes in water quality parameters, which may affect growth.

By realizing the overriding significance of water chemistry, it is important to have a firm grasp of some basic concepts. Like:

Temperature

Aquaculture organisms are cold-blooded animals. They can modify their body temperature to the environment in normal condition, unlike the warm-blooded animals, which can react to maintain the optimum body temperature. For eg. the optimum range of temperature for the Black Tiger shrimp is between 28°C-30°C. Increase in temperature beyond 30°C increases the activity level and the metabolism. This also increases the growth rate. If the temperature still increases then the shrimp reaches a threshold of physical and nutritional tolerance, which is 33°C in poor quality water or 35°C in good quality water and remains stationary at the pond bottom.

If the environment does not improve the culture organisms may get infected by germs, swim in a disoriented way to the surface or due to exhaustion. If the temperature falls below 28°C, the metabolism reduces and so does the active behaviour and growth rate. Below 20°C, the shrimp will take less feed. Shrimps cannot tolerate a temperature less than 13°C.

In the semi intensive culture system, shrimps are more sensitive to temperature than in the extensive one because of the higher biomass and less water volume. During the rainy season, there is a greater possibility of occurrence of thermal stratification in pond water column, as well as the salinity (density) and dissolved oxygen stratification.

Water depth and water volume affect the thermal capacity of the pond and the extent of light penetration. It is related to fluctuation of planktonic algae and benthic algae.

It also influences the volume of the pond and therefore the ponds capacity to support the dissolved oxygen, influencing productivity, biomass and production yield.

Salinity

Salinity plays an important role in the growth of culture organisms through osmoregulations of body minerals from that of the surrounding water. For eg. the optimum range of salinity for black tiger shrimp is between 10 and 25 ppt, although the shrimp will accept salinity between 5 and 38 ppt. since its eurihaline character. The early life stages of both shrimp and prawn requires standard seawater salinities but while growing they can with stand to brackishwater or even to freshwater. However, for better survival and growth optimum range of salinity should be maintained in the aquaculture ponds.

Oxygen

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PH (measure of acidity or alkalinity)

PH or the concentrations of hydrogen ions (H+) present in pond water is a measure of acidity or alkalinity. The pH scale extends from 0 to 14 with 0 being the most acidic and 14 the most alkaline. PH 7 is a condition of neutrality and routine aquaculture occurs in the range 7.0 to 9.0 (optimum is 7.5 to 8.5). Exceedingly alkaline water (greater than pH 9) is dangerous as ammonia toxicity increases rapidly. At higher temperatures fish are more sensitive to pH changes.

It is an important chemical parameter to consider because it affects the metabolism and other physiological processes of culture organisms. A certain range of pH (pH 6.8 – 8.7) should be maintained for acceptable growth and production. But in semi- intensive culture, re-optimum range is better maintained between pH 7.4 – 8.5. pH 7 is the neutral point and water is acidic below pH 7 and basic above pH 7. pH changes in pond water are mainly influenced by carbon dioxide and ions in equilibrium with it. PH can also be altered by a) Organic acids, these are produced by anaerobic bacteria (“acid formers”) from protein, carbohydrates and fat from feed wastes, b) Mineral acids such 7 as sulfuric acid (acid-sulfate soils), which may be washed down from dikes during rains and c) Lime application.

Like DO, a diurnal fluctuation pattern that is associated with the intensity of photosynthesis, occurs for pH. This is because carbon dioxide is required for photosynthesis and accumulates through nighttime respiration. It peaks before dawn and is at its minimum when photosynthesis is intense. All organisms respire and produce Carbon dioxide (CO2) continuously, so that the rate of CO2 production depends on the density of organisms. The rate of CO2 consumption depends on phytoplankton density. Carbon dioxide is acidic and it decreases the pH of water. Also, at lower pH, CO2 becomes the dominant form of carbon and the quantity of bicarbonate and carbonate would decrease. The consumption of CO2 during photosynthesis causes pH to peak in the afternoon and the accumulation of CO2 during dark causes pH to be at its minimum before dawn.

The pH should be monitored before dawn for the low level and in the afternoon for the high level. The magnitude of diurnal fluctuation is dependent upon the density of organisms producing and consuming CO2 and on the buffering capacity of pond water (greater buffer capacity at higher alkalinity). i.e., Diurnal fluctuation of pH is not great in pond water of higher alkalinity. An alkalinity above 20 ppm CaCO3 is preferred in prawn/shrimp ponds. Intervention, such as flushing of ponds to reduce the pH, is advisable when the magnitude of diurnal fluctuation in pH is great.

Nevertheless, one should notice that the drastic fluctuation of pH would cause stress to culture organisms. Normally, it should maintain the daily fluctuation within a range of 0.4 difference. Control of pH is essential for minimizing ammonia and H2S toxicity.

Ammonia

Ammonia is the second gas of importance in fish culture; its significance to good fish production is overwhelming. High ammonia levels can arise from overfeeding, protein rich, excess feed decays to liberate toxic ammonia gas, which in conjunction with the fishes, excreted ammonia may accumulate to dangerously high levels under certain conditions. Fortunately, ammonia concentrations are partially ‘curbed’ or ‘buffered’ by conversion to nontoxic nitrate (No3 -) ion by nitrifying bacteria. Additionally, ammonia is converted from toxic ammonia (NH3) to nontoxic ammonium ion (NH4 +) at pH below 8.0.

Hardness

Numerous inorganic (mineral) substances are dissolved in water. Among these, the metals calcium and magnesium, along with their counter ion carbonate (CO3 -2) comprise the basis for the measurement of ‘hardness’. Optimum hardness for aquaculture is in the range of 40 to 400 ppm of hardness. Hard waters have the capability of buffering the effects of heavy metals such as copper or zinc which are in general toxic to fish. The hardness is a vital factor in maintaining good pond equilibrium.

Turbidity

Water turbidity refers to the quantity of suspended material, which interferes with light penetration in the water column. In prawn ponds, water turbidity can result from planktonic organisms or from suspended clay particles. Turbidity limits light penetration, thereby limiting photosynthesis in the bottom layer. Higher turbidity can cause temperature and DO stratification in prawn ponds.

Planktonic organisms are desirable when not excessive, but suspended clay particles are undesirable. It can cause clogging of gills or direct injury to tissues of prawns. Erosion or the water itself can be the source of small (1-100 nm) colloidal particles responsible for the unwanted turbidity. The particles repel each other due to negative-charges: this can be neutralized by electrolytes resulting in coagulation. It is reported that alum and ferric sulfate are more effective than hydrated lime and gypsum in removing clay turbidity. Both alum and gypsum have acid reactions and can depress pH and total alkalinity, so the simultaneous application of lime is recommended to maintain the suitable range of pH. Treatment rates depend on the type of soil.

Redox Potential (Oxidation-Reduction Eh)

Redox Potential is an index indicating the status of oxidation or reduction. It is correlated with chemical substances, such as O2, CO2 and mineral composed of aerobic layer, whereas H2S, CO2, NH3, H2SO4 and others comprise of anaerobic layer. Microorganisms are correlated with the status of oxidation or reduction. With the degree of Eh, it is indicative of one of the parameters that show the supporting ability of water and soil to the prawn biomass.

In semi intensive culture photosynthetic bacteria (PSB) plays an important role through absorption and conversion of organic matter into the minerals and nutrients as a secondary production, compared to the primary production of algal population. PSB exist particularly due to low oxygen level and high intensity of light and can significantly improve the culture environment.

Water quality management

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Water Exchange

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Removal of dissolved metabolic organics

One of the important stress factors is the increase of dissolved metabolic organics in culture water. It can increase ammonia and microorganisms.

This explains why water quality deterioration could quickly cause a high mortality rate. To prevent the buildup of dissolved organics, frequent partial to total water change is necessary; or the pollution could be reduced by the chemically removing the pollutants by adsorption using activated carbon.

The best way to facilitate the removal of metabolic wastes in a pond is by flushing out water from the bottom. Constantly maintaining high DO in the pond through supplemental aeration and water exchange, enhances nitrification. Nitrification is a major mechanism for ammonia removal in well-aerated ponds. Paddlewheel aerators are usually operated during dark (7 pm to 7 am) when oxygen depletion is likely to occur and at noon (12 noon to 2 pm) when temperature and oxygen stratification can become significant.

Phytoplankton management

Phytoplankton play a significant role in stabilizing the whole pond ecosystem and in minimizing the fluctuations of water quality. A suitable phytoplankton population enriches the system with oxygen through photosynthesis during day light hours and lowers the levels of CO2, NH3, NO2 and H2S. A healthy phytoplankton bloom can reduce toxic substances since phytoplankton can consume NH4 and tie-up heavy metals. It can prevent the development of filamentous algae since phytoplankton can block light from reaching the bottom. A healthy bloom also provides proper turbidity and subsequently stabilizes shrimp and reduces cannibalism. It decreases temperature loss in winter and stabilizes water temperature.

Pond bottom treatment

For farms adopting advanced technology, it is necessary that pond bottom should be completely dried and aerated to get rid of toxic gases.

Many ponds in low-lying areas cannot be completely drained and dried. To overcome this, Aquafarmers apply waste digesters to the ponds. The digesters are harmless bacteria (probiotics) and enzymes that consume organic matter on the pond bottom.

After the application of digesters farmers apply a disinfectant, either organic silver or organic iodine. Copper sulphate is not used as a disinfectant now a days as it is not biodegradable and accumulates in the pond upto levels that are toxic to aquatic life. Organic silver is highly effective against bacteria and viruses and its toxicity to aquatic life is very low. Organic silver is applied at the rate of 18 litres (4 gallon) per hectare after lowering the water depth to 12 inches. Seven days after the application, this disinfectant disintegrates, so there is no need to flush the pond. Organic silver also prevents the development of algae that grows on shells.

Organic iodine, can cure gill or shell diseases, kills bacteria on contact and has low toxicity. Its effect can be noticed within 24 hours and the pond bottom can be disinfected without emptying the pond. The suggested dosage is 5 ppm to 10 ppm. Its affectivity lasts for two to three days compared to about seven days in the case of organic silver.

Nitrogen Metabolites

Large quantities of organic matter originating from the heavy feed load and feacal matter accumulate in aquaculture ponds. These undergo oxidation-reduction reactions leading to decomposition, mainly through the action of bacteria. Different forms of inorganic nitrogen like ammonia, nitrite and nitrate are produced during decomposition.

Maintaining water quality and preventing diseases

Environmental conditions vary considerably at different times of the year and the bacterial and fungal, load of seawater also varies. During the dry moths; there is less dilution of organic and toxic pollutants from human and industrial wastes. During this time the absence of rains also reduces water exchange between clean seawater and polluted coastal water. The result is a rise of viral, bacteria, protozoa, fungi and toxic pollutants in the water. This is partially upset during the hot summer months by phytoplankton and zooplankton blooms, which assimilate some of the bacteria and toxic substances. Under such conditions, cultured animals become vulnerable to infection. They are stressed by the following:

Overcrowding in captivity.

•  Temperature fluctuation of water, especially during water change (A onedegree Celsius difference can cause stress).

•  A temporary decline in dissolved oxygen level due to power failure.

•  Increase of free-carbon dioxide, un-ionized ammonia and organics due to decaying excess feed and dead animals.

•  Physical manhandling during water change.

•  Poor nutrition – improperly fed fish and prawn.

•  The high level of toxic pollutant in seawater that may contain heavy metals such as copper, zinc, lead, nickel, mercury and chemicals like poly-chlorinated biphenyl compounds, chlorinated hydrocarbons such as DDT and other pesticides.

While there is no known practical way to remove these pollutants. Effect should be made to limit these stress-inducing factors to keep the animals strong enough to fight infection. Healthy animals, do not easily succumb to diseases. Where adequate filtration is not possible, treatment of water is suggested to lower the bacteria and fungal load of the water.

Self-pollution as a possible factor

When the accumulation of nutrients within ponds is high, self-pollution of the culture environment reduces production, frequently as a result of a severe disease-outbreaks. Although, in some cases, production losses can be linked back directly to diseaseoutbreaks, it is often difficult to separate the effect of disease and poor water quality. Disease-outbreaks occur when (1) a pathogen infects a population previously not exposed to the microorganism. or (2) poor culture conditions weaken resistance to pathogens permanently present in the culture environment. Outbreaks of new infectious diseases will be difficult to prevent as long as there are no strict regulations for transfer of culture stocks between regions. From a practical point of view, more attention should be paid to culture conditions, with special attention to water quality.

Some farms, experienced a collapse in production from 15-18 Mt ha-1 to 4-6 Mt ha-1, but were able to restore production levels of 10-12 Mt ha-1 year-1 on a continuous basis. These farmers concentrated on water quality management, introducing measures such as high levels of pond flushing (>30% day-1) excavation and tilling of pond bottoms upon harvest, emergency aeration and the use of drugs, chemicals and biological agents to suppress disease-outbreaks. Farmers apply these measures empirically.

Details of the importance of physico-chemical parameter and microbiological aspects in aquaculture ponds.

#	Parameter	Purpose	Equipment Required	Chemicals and Glassware Required
I. Water quality parameters
1	Temperature	Maintenance of optimal temperature, fluctuations at high level, leeds to severe effect on entire body of pond and leeds thermal stress on shrimp, algal crash etc. (28 - 32°C)	Mercuric thermometer / Digital thermometer	---
2	Salinity	Eurihaline; tolerance capacity with broad range of salinity (10 - 25 ppt)	Clinometers (Refract meter)	Titrimetric method of knewdson’s for standardization of salinometer
3	PH	Little bit basic conditions are favourable (7.4 – 8.5)	Digital PH meter / pH pen	Standardized by Titrimetric method
4	Transparency (Độ trong)	Optimal maintenance of plankton density (30 - 40 cm)	Sacchi disc	Primary productivity estimation by dark white bottle method by C14 method
5	Dissolved Oxygen	Is the main requirement for physiological & biochemical activities of living things (> 3 mg/l)	D.O. Meter/ Wrinklers method	Chemical and Glassware required burette, pipette, conical flasks, D.O bottles (reagent bottles), beakers etc.
6	BOD	It is necessary to estimate the requisite of oxygen content by the enclosed body of water	D.O. Meter/ Wrinklers method	D.O method/ WrinklerChemical and Glassware required burette, pipette, conical flasks, D.O bottles(reagent bottles), Beakers
7	COD	The amount of oxygen required for the complete oxidation of all organic/chemical components in the pond environment	Oxidized method	Chemical & Glassware required reflexor, hotplate etc.
8	Nitrate – N	The nutrient content is very high leads to heavy blooming of algae	Spectrophotometer meter/ Calorimeter/kits (YSI, Merck, Loba)	Chemicals & Glassware
9	Nitrate – N	It is an toxic constituent	Spectrophotometer/ Calorimeter/ kits (YSI, Merck, Loba etc.)	Chemicals & Glassware
10	Ammonia–N	It is an toxic constituent	Spectrophotometer/ Calorimeter/ kits	Chemicals & Glassware
11	Phosphate–P	Nutrient	Spectrophotometer/ Calorimeter/ kits	Chemicals & Glassware
12	Hydrogen sulphide	Toxic and makes anoxic conditions	Spectrophotometer/ Calorimeter/ kits	Chemicals & Glassware
13	Sediment analysis a. Sediment organic matter	It shows how much of organic matter and organic carbon produced in the pond bottom by the shrimp farming activity.	Titrimetric method	Chemicals & Glassware
	b. Sediment composition	It exhibits the ratio’s of sediment components i.e., sand, silt & clay	Pipette method	Chemicals & Glassware
II. Biological
	A. Bacteriological
1	TVC (Total Viable count of Bacteria)	It reveals the total bacterial forms that harbour the pond environment	Laminar flow chamber, oven, incubator, autoclave, petridishes, test tubes, conical flasks, test tube stand, micropipette, beakers, digital colony counter.	TGY media (Tryptone Glucose Yeast extract Agar)
2	TVLO (Total Vibrio like organizers)	It reveals the total bacterial forms that harbour the pond environment	Laminar flow chamber, oven, incubator, autoclave, petridishes, test tubes, conical flasks, test tube stand, micropipette, beakers, digital colony counter.	TCBS media (Thiamine Citrate, Bile sucrose salt Agar)
3	Other pathogenic bacteria	It reveals the total bacterial forms that harbour the pond environment	-DO-	DODifferent media for respective various bacteria
	B. Virology
1	Dot Blot method	Viral diseases like WSV, MBV etc.	Kits	--
2	PCR method	Viral diseases like WSV, MBV etc.	Primers, Thermal cyclar, UV laminar flow chamber, electrophoresis unit, UPS system etc.	--
3	Histopathology	Viral diseases like WSV, MBV etc.	Microtome, staining & distaining equipment and stains.	--
C.	Ectocommensal Parasites		Like Zoothamnium, Vorticella, Fungi etc.	Simple binocular microscope and Compound monocular microscope with CCTV.

Details of the analysis methods and the required equipment for the physico-chemical parameters and microbiological aspects of water and sediment in aquaculture ponds. 

Standards of Water Quality for Aquaculture

ITEMS	VALUE OF STANDARD
Colour, offensive smell	Fish, shrimp, shell fish and kelp should not have odd colour, odd offensive smell.
Floating material	No oil film and floating foam should appear on the water surface
Suspended material (mg/l)	The amount added to human beings should not surpass 10, and the suspending materials sunk to the bottom of the water should not be harmful to fish, shrimp and shellfish
pH value	Freshwater 6.5-8.5, seawater 7.4-8.5
Dissolved oxygen	In successive 24h, above 16 h should be higher than 5 mg/l, and the other time should not be lower than 3 mg/l
Biochemical Oxygen Demand (5 days, 20°C)	Should not surpass 5 mg/l, frozen period should not surpass 3 mg/l
Total colonial bacillus	Should not be greater than 5000/L (and should not surpass 500 pieces/L)

Water quality parameters for shrimp farming

Water parameter	Optimum level
Temperature	26-33 C
Salinity	10-25 ppt
Dissolved oxygen	>4.0 ppm
pH	7.5-8.5
Total Ammonia Nitrogen	<1.0 ppm
Total Nitrate Nitrogen	<5.0 ppm
Nitrite Nitrogen	<0.01 ppm
Sulphide	<0.03 ppm
Biological Oxygen Demand (BOD)	< 10 ppm
Chemical Oxgen Demand (COD)	25-45 cm
Sacchi disc visibility