Aquatic environment and water quality

In short

The aquatic environment is the physical, chemical and biological setting in which cultured fish and shellfish live; water quality is the set of measurable variables in that setting that decide survival, feed intake, growth and disease resistance. In aquaculture, water is simultaneously the animal's living medium, its oxygen source and its waste sink, so almost every production failure — slow growth, mass mortality, poor FCR — traces back to a water-quality variable crossing a threshold. The topic is examined in B.F.Sc core aquaculture papers, in ICAR AIEEA-PG and ASRB NET, in ARS/NET fisheries resource-management sections, and in State Fisheries Department and MPEDA recruitment tests. On the job it underpins daily pond monitoring in Andhra Pradesh and West Bengal farms, hatchery water treatment, CAA-registered farm compliance, effluent monitoring under CPCB norms and the water-quality sections of NFDB project reports.

Explained

Physical variables

• Temperature: controls metabolic rate, oxygen demand and solubility. Most Indian warm-water species (carps, tilapia, catfish) grow best at 26–32 °C; trout (Himalayan farms) need 10–18 °C. A rule of thumb: metabolic rate roughly doubles per 10 °C rise (Q₁₀ ≈ 2).
• Fish are poikilothermic (ectothermic) — body temperature follows water, so feeding is cut in winter and in heat waves.
• Thermal stratification: in ponds deeper than ~1.5–2 m, a warm epilimnion sits over a cold hypolimnion separated by a thermocline/metalimnion. The lower layer goes anoxic; sudden mixing (overturn, after cold rain or storm) brings up oxygen-free, sulphide-rich water and kills fish.
• Indian ponds are polymictic (mix often); deep reservoirs are typically warm monomictic.
• Light and transparency: measured by Secchi disk. Ideal grow-out transparency 25–40 cm; <20 cm means a dense bloom (night-time DO crash risk), >60 cm means poor productivity and benthic algal mats. Euphotic zone depth ≈ 2–3 × Secchi depth.
• Turbidity: clay turbidity (silt) blocks light and abrades gills — treated with gypsum, alum or organic manure; plankton turbidity is desirable in moderation.
• Colour: green/brown-green = healthy phytoplankton; dark brown-black = decaying organic matter; blue-green scum = cyanobacterial bloom; milky white = zooplankton bloom or dinoflagellate crash.
• Depth and area: carp grow-out ponds 1.5–2.0 m water depth; nursery ponds 1.0–1.5 m; shrimp ponds 1.0–1.5 m.

Dissolved gases

• Dissolved oxygen (DO) is the single most critical variable. Sources: photosynthesis (largest), atmospheric diffusion, aeration, water exchange. Sinks: fish and plankton respiration, sediment oxygen demand, chemical oxidation of ammonia and sulphide.
• Safe range: ≥5 mg/L for good growth; 3–5 mg/L = stress, reduced feeding; <2 mg/L = emergency; <1 mg/L prolonged = mortality. Shrimp (*Penaeus vannamei*) needs >4 mg/L at the pond bottom.
• Diurnal DO cycle: minimum just before dawn (04:00–06:00, respiration only all night), maximum mid-afternoon (14:00–16:00). Monitoring at dawn and dusk is standard farm practice.
• Oxygen solubility falls with rising temperature and salinity, and with altitude (lower pressure). Air-saturation DO ≈ 8.3 mg/L at 25 °C and ≈ 7.5 mg/L at 30 °C in freshwater.
• Supersaturation (>100%) from heavy blooms can cause gas bubble disease; so can air leaks on pump suction lines.
• Carbon dioxide: respiration product; high CO₂ (>10–15 mg/L) lowers pH and causes internal hypercapnia — blood pH falls, haemoglobin's oxygen affinity drops (Root/Bohr effect), so fish suffocate even with adequate DO.
• Ammonia (NH₃/NH₄⁺): excreted across gills; the un-ionised NH₃ fraction is the toxic one.
• Hydrogen sulphide (H₂S): formed by sulphate-reducing bacteria in anoxic black sediment; toxic above 0.002 mg/L as un-ionised H₂S, and more of it is un-ionised at low pH (opposite to ammonia). Detected by rotten-egg smell and black, greasy pond bottom.
• Methane in reduced sediment indicates severe organic overload.

pH, alkalinity, hardness and the carbonate buffer

• pH: safe range 6.5–9.0, optimum 7.0–8.5. Below 5 or above 10 is lethal; acid-sulphate soils of coastal West Bengal, Kerala and Odisha give chronically low pH.
• Diurnal pH swing: photosynthesis strips CO₂, so pH rises through the day (can touch 9–10 in a bloom) and falls at night. A swing of more than ~1 unit indicates weak buffering.
• Total alkalinity: capacity to neutralise acid, mainly HCO₃⁻ and CO₃²⁻, expressed as mg/L CaCO₃. Productive aquaculture water: 75–200 mg/L; below 20 mg/L is poorly buffered; shrimp ponds are run at 100–150 mg/L (vannamei, higher for moulting).
• Total hardness: sum of divalent cations, mainly Ca²⁺ and Mg²⁺, also as mg/L CaCO₃. Needed for bone, scale and shell formation and for osmoregulation; >50 mg/L desirable, 20 mg/L minimum.
• Alkalinity and hardness are usually similar in natural waters; wide divergence signals unusual geology or amendment.
• Liming raises pH, alkalinity and hardness, kills parasites, flocculates clay and releases bound phosphorus. Materials in order of neutralising value: quicklime CaO (~179%) > hydrated lime Ca(OH)₂ (~136%) > dolomite CaMg(CO₃)₂ (~109%) > agricultural limestone CaCO₃ (100%).
• Indicative dose for acidic ponds: 200 kg/ha (pH 6.5–7.5) to 1,000–2,000 kg/ha (pH <5) of agricultural lime; split doses, and never apply quicklime with fish in the pond.
• Gypsum (CaSO₄·2H₂O) raises hardness without raising pH — used when hardness is low but pH is already high.

Nitrogen, phosphorus and the biological cycle

• Nitrogen cycle in a pond: feed protein → fish excretion and decomposition → TAN (total ammonia nitrogen) → *Nitrosomonas* → nitrite (NO₂⁻) → *Nitrobacter/Nitrospira* → nitrate (NO₃⁻) → uptake by algae, or denitrification to N₂ in anoxic zones. Nitrification consumes oxygen (≈4.57 g O₂ per g N) and alkalinity (≈7.14 g CaCO₃ per g N).
• Toxicity order: un-ionised NH₃ > NO₂⁻ > NO₃⁻. Safe limits: un-ionised NH₃-N <0.1 mg/L (chronic; <0.025 mg/L preferred for sensitive stages), NO₂⁻-N <0.5 mg/L (<1 mg/L in brackish water, where chloride is protective), NO₃⁻-N <50–100 mg/L.
• Ammonia toxicity symptoms: gill hyperplasia, reduced oxygen uptake, lethargy, loss of equilibrium; nitrite causes methaemoglobinaemia ("brown blood disease") — Fe²⁺ oxidised to Fe³⁺, blood cannot carry oxygen. Remedy: add salt (NaCl); Cl⁻ competes with NO₂⁻ at the gill.
• Un-ionised fraction rises with pH and temperature — the same TAN is harmless at pH 7 and lethal at pH 9.
• Phosphorus is usually the limiting nutrient for primary production in Indian freshwater ponds. Soluble reactive phosphate 0.05–0.2 mg/L sustains a bloom; it adsorbs strongly on acidic clay, so liming before fertilising is essential.
• N:P ratio of about 4:1 to 10:1 is used for fertiliser scheduling; common Indian practice is cattle manure 10,000–15,000 kg/ha/yr plus urea and single superphosphate in split doses.
• Organic load indicators: BOD₅ (biochemical oxygen demand over 5 days at 20 °C) and COD. Pond water BOD should stay <10 mg/L; COD/BOD ≈ 2 for readily biodegradable waste.
• Plankton: desirable phytoplankton — *Chlorella*, *Scenedesmus*, *Navicula*, diatoms; undesirable — *Microcystis*, *Anabaena*, *Oscillatoria* (cyanobacteria; toxins, off-flavour, night DO crash). Zooplankton (*Rotifers*, *Moina*, *Daphnia*, copepods) are nursery live feed. Plankton volume 1–3 mL per 50 L is a practical target.

Salinity, ions and brackish-water parameters

• Salinity in ppt (g/kg) or as conductivity (µS/cm). Freshwater <0.5 ppt; brackish 0.5–30 ppt; seawater ~35 ppt.
• *Penaeus vannamei* is euryhaline: 10–25 ppt optimum, survives 0.5–40 ppt with acclimatisation; *P. monodon* 15–25 ppt; carps <5 ppt.
• For low-salinity inland vannamei culture, ionic balance matters more than salinity: Na:K ≈ 28:1 and Mg:Ca ≈ 3:1, with K⁺ >8–11 mg/L per ppt (seawater has ~11 mg/L K per ppt); potassium deficiency causes mortality at moulting.
• Osmoregulation: freshwater fish are hyperosmotic to the medium — they absorb water, excrete dilute urine copiously and take up ions at gills; marine teleosts are hypoosmotic — they drink seawater and excrete salt via chloride cells. Isosmotic point for most teleosts is ~8–12 ppt, where osmoregulatory energy cost is least.
• Acclimatisation rule for seed: change salinity by not more than 3–5 ppt per hour and temperature by not more than 1–2 °C per 15 minutes during transport and stocking.
• Iron (<0.5 mg/L), hydrogen sulphide, residual chlorine (<0.003 mg/L) and heavy metals (Cu, Zn, Cd, Pb) must be checked in borewell and creek source water.

Management, measurement and Indian regulation

• Aeration: paddle-wheel (shrimp and intensive ponds), aspirator/venturi, diffused air, air-lift. Indicative requirement 1 HP per 400–500 kg of standing biomass; vannamei ponds commonly run 4–8 HP/ha. Aerators also create a central sludge mound for removal.
• Other interventions: water exchange (now minimised for biosecurity), probiotics and bioremediation (*Bacillus* spp.), molasses/carbon dosing to raise the C:N ratio (~12–15:1) and immobilise ammonia as microbial protein (biofloc principle), zeolite for ammonia adsorption, KMnO₄ for organic load, and bottom drying/ploughing between crops.
• Measurement methods: DO by Winkler iodometric titration (azide modification) or membrane/optical DO probe; pH by potentiometry; alkalinity by titration with 0.02 N H₂SO₄ to methyl orange/pH 4.5 end point; hardness by EDTA titration with Eriochrome Black T; ammonia by phenate (indophenol-blue) or Nessler method; nitrite by diazotisation (sulphanilamide–NED); BOD by 5-day incubation at 20 °C. Reference text for methods: APHA Standard Methods; Indian water-sampling and test methods are covered by the IS 3025 series (water and wastewater methods of sampling and test) and drinking water by IS 10500:2012.
• Regulation: the Coastal Aquaculture Authority Act, 2005 (as amended in 2023) requires registration of coastal aquaculture farms, bans aquaculture in CRZ No-Development Zones, prohibits use of antibiotics and pharmacologically active substances listed in the Schedule, and mandates effluent treatment systems (ETS) on 10% of farm area for farms above ~5 ha, with discharge standards for BOD, suspended solids, pH, phosphate and dissolved nitrogen.
• CPCB/MoEFCC general effluent discharge norms commonly cited for inland surface water: pH 5.5–9.0, BOD₅ ≤30 mg/L, suspended solids ≤100 mg/L, oil and grease ≤10 mg/L — verify against the current Environment (Protection) Rules schedule, since category-specific limits are revised.
• Other instruments: EIA Notification 2006 (as amended) for larger projects, the Water (Prevention and Control of Pollution) Act, 1974 for discharge consent, and MPEDA/NaCSA and NFDB guidelines for Better Management Practices. Fish and fishery products for export are additionally tested for residues under EIC/MPEDA monitoring plans.

Key points

• Water quality - the physical, chemical and biological properties of culture water that limit survival and growth; the medium is also the waste sink.
• Dissolved oxygen - ≥5 mg/L target; dawn minimum, afternoon maximum; solubility falls with temperature, salinity and altitude.
• Diurnal cycle - DO and pH both peak in the afternoon and bottom out at dawn, driven by photosynthesis versus respiration.
• Secchi disk - transparency meter; 25–40 cm is the working window for fed carp and shrimp ponds.
• Stratification and overturn - anoxic hypolimnion mixing upward is a classic cause of sudden mass mortality.
• Un-ionised ammonia - the toxic species; its share of TAN rises sharply with pH and temperature; keep NH₃-N <0.1 mg/L.
• Nitrite - causes brown-blood disease (methaemoglobinaemia); countered by chloride (common salt).
• Nitrification - two-step bacterial oxidation that consumes 4.57 g O₂ and 7.14 g CaCO₃ alkalinity per g of ammonia-N.
• Total alkalinity - buffering capacity as mg/L CaCO₃; 75–200 mg/L keeps pH swings small.
• Total hardness - Ca²⁺ + Mg²⁺ as mg/L CaCO₃; needed for shell, bone and osmoregulation.
• Liming - raises pH, alkalinity, hardness; neutralising value CaO > Ca(OH)₂ > dolomite > CaCO₃.
• Gypsum - raises hardness/calcium without raising pH.
• H₂S - lethal above ~0.002 mg/L un-ionised; more toxic at low pH, unlike ammonia.
• CO₂ - high levels cause blood acidosis and reduce haemoglobin oxygen loading even at adequate DO.
• Salinity and ionic ratios - Na:K ≈ 28:1, Mg:Ca ≈ 3:1 are critical for low-salinity inland vannamei culture.
• Aeration - roughly 1 HP per 400–500 kg biomass; paddle wheels also concentrate sludge centrally.
• CAA Act, 2005 (amended 2023) - farm registration, banned substances, effluent treatment system on ~10% of farm area.
• Measurement - Winkler for DO, EDTA for hardness, phenate for ammonia, acid titration for alkalinity; IS 3025 series and APHA methods.

Formulas and examples

Example 1 — Un-ionised ammonia at two pH values. A carp pond at 25 °C has TAN = 2.0 mg/L.

• Morning pH 7.4: pKa − pH = 9.24 − 7.4 = 1.84 → 10^1.84 = 69.2 → f = 1/70.2 = 0.0142.
• NH₃-N = 2.0 × 0.0142 = 0.028 mg/L — just acceptable.
• Afternoon pH 9.2: pKa − pH = 0.04 → 10^0.04 = 1.096 → f = 1/2.096 = 0.477.
• NH₃-N = 2.0 × 0.477 = 0.95 mg/L — roughly 34 times higher and clearly toxic.
• Conclusion: report ammonia with pH and temperature; afternoon sampling in a dense bloom is the worst case.

Example 2 — Lime requirement for an acidic pond. A 0.8 ha pond with acid soil needs a lime requirement of 1,500 kg/ha as pure CaCO₃. The farmer has dolomite (neutralising value 109%).

• Quantity as CaCO₃ = 1,500 × 0.8 = 1,200 kg.
• Dolomite needed = 1,200 × (100/109) = 1,101 kg ≈ 1.1 t.
• At ₹9 per kg delivered, cost ≈ ₹9,900 for the pond. Apply in two split doses over the dry pond bottom, 15 days before filling.

Example 3 — Salt dose to counter nitrite. A 0.5 ha pond, mean depth 1.2 m, shows NO₂⁻-N = 1.8 mg/L. Target chloride about 100 mg/L.

• Volume = 5,000 m² × 1.2 m = 6,000 m³.
• Chloride needed = 100 mg/L × 6,000 m³ / 1000 = 600 kg of Cl⁻.
• NaCl is 60.66% chloride by mass → NaCl = 600 / 0.6066 = 989 kg ≈ 1,000 kg (1 t) of common salt.
• At ₹8 per kg, cost ≈ ₹8,000; also stop feeding for a day and raise aeration.

Example 4 — Nitrification load from feed. A vannamei pond gets 300 kg/day of 35% crude protein feed.

• Nitrogen fed = 300 × 0.35 / 6.25 = 16.8 kg N/day.
• Assume 25% retained in shrimp → N entering water = 16.8 × 0.75 = 12.6 kg N/day.
• Oxygen for complete nitrification = 12.6 × 4.57 = 57.6 kg O₂/day.
• Alkalinity destroyed = 12.6 × 7.14 = 90.0 kg CaCO₃/day — in a 1 ha pond at 1.2 m (12,000 m³) that is 90/12 = 7.5 mg/L per day, which is why alkalinity must be topped up with dolomite or sodium bicarbonate.

Example 5 — Winkler titration. A 300 mL BOD bottle sample is titrated with 0.025 N sodium thiosulphate; 3.6 mL is used (whole bottle contents titrated).

• DO = (3.6 × 0.025 × 8 × 1000) / 300 = 720/300 = 2.4 mg/L.
• At 30 °C saturation is about 7.5 mg/L, so saturation = 2.4/7.5 = 32% — a dawn emergency; start all aerators and withhold feed.

Where it is used

• Grow-out farms: daily DO, pH, transparency and alkalinity logs on carp farms in Andhra Pradesh (Kolleru belt), West Bengal and Punjab, and on vannamei farms in Nellore, Bapatla and Nagapattinam — the farm technician's core routine.
• Hatcheries: source-water treatment (sand filtration, cartridge filters, UV, chlorination–dechlorination), temperature and salinity control for carp, prawn and shrimp hatcheries; water-quality logs required for CAA/MPEDA hatchery registration.
• Recirculating aquaculture systems (RAS) and biofloc: mechanical and biofilter design, TAN–nitrite monitoring, alkalinity dosing — growing roles in Kerala, Telangana and Haryana RAS units.
• Regulation and testing laboratories: ICAR institutes (CIFA Bhubaneswar, CIBA Chennai, CIFE Mumbai, CIFRI Barrackpore), State Fisheries Department labs, CAA/CPCB compliance monitoring, NFDB-funded project appraisal.
• Job roles: Farm/Aqua Technician, Water Quality Analyst, Hatchery Manager, Aqua-feed company technical service officer (Avanti, CP India, Growel, Godrej Agrovet), Fisheries Development Officer, Fisheries Extension Officer, Fisheries Inspector.
• Allied use: reservoir and wetland fisheries management, cage culture site selection in reservoirs, effluent treatment design for processing plants, and ornamental fish retail.

Interview questions

• Q: Why is dissolved oxygen lowest at dawn in a fish pond? A: There is no photosynthesis at night, while fish, plankton, bacteria and the sediment keep respiring, so oxygen is drawn down continuously from dusk to sunrise.
• Q: The lab reports TAN 3 mg/L. Is the pond in danger? A: Cannot say without pH and temperature, since only un-ionised NH₃ is toxic — 3 mg/L TAN is harmless at pH 7 but gives nearly 1.4 mg/L NH₃-N at pH 9 and 30 °C.
• Q: What is brown-blood disease and how do you treat it? A: Nitrite oxidises haemoglobin iron to methaemoglobin, which cannot carry oxygen; adding common salt supplies chloride that competes with nitrite at the gill, alongside aeration and reduced feeding.
• Q: Difference between alkalinity and hardness? A: Alkalinity measures acid-neutralising anions (mainly bicarbonate and carbonate); hardness measures divalent cations (mainly calcium and magnesium) — both expressed as mg/L CaCO₃, which is why they are confused.
• Q: Why is H₂S more toxic at low pH while ammonia is more toxic at high pH? A: Both are weak-acid/base pairs — low pH keeps sulphide in the un-ionised H₂S form, while high pH shifts ammonium to un-ionised NH₃; the un-ionised molecules cross the gill membrane.
• Q: Why does the Secchi reading matter to a shrimp farmer? A: It is a one-minute proxy for plankton density, hence for night-time oxygen demand and light penetration; below 20 cm he must cut feed and fertiliser and increase aeration.
• Q: What is pond overturn? A: Sudden destratification — usually after cold rain or a storm — mixing deoxygenated, sulphide-laden bottom water through the pond, often causing overnight mass mortality.
• Q: How much aeration does a 1 ha vannamei pond with 8 t standing biomass need? A: At roughly 1 HP per 400–500 kg biomass, about 16–20 HP, generally provided as 8–10 paddle wheels of 2 HP, positioned to sweep sludge to the centre.
• Q: Why does intensive culture consume alkalinity? A: Nitrifying bacteria destroy about 7.14 g of alkalinity as CaCO₃ per gram of ammonia-nitrogen oxidised, so bicarbonate must be replenished with dolomite or sodium bicarbonate.
• Q: In low-salinity inland vannamei culture, what kills shrimp even when salinity looks fine? A: Ionic imbalance — mainly potassium and magnesium deficiency (Na:K far above 28:1), which shows up as mortality at moulting.
• Q: What does the Coastal Aquaculture Authority Act require on effluent? A: Registered coastal farms above about 5 ha must provide an effluent treatment system on roughly 10% of the farm area and meet prescribed discharge limits; the Act also bans scheduled antibiotics and prohibits farming in CRZ no-development zones.
• Q: How would you fix a pond with pH 5.8, alkalinity 15 mg/L and poor plankton? A: Lime first — agricultural limestone or dolomite by soil lime requirement — because fertiliser phosphorus is locked by acidic clay; only after alkalinity reaches 60–80 mg/L should manure and phosphate be applied.

Common mistakes

• Quoting TAN or "ammonia" as a toxicity figure without pH and temperature, and so missing a lethal un-ionised NH₃ level.
• Treating alkalinity and hardness as the same thing because both are reported as mg/L CaCO₃.
• Sampling DO at 10 a.m. or in the afternoon and concluding the pond is safe, when the dawn minimum is the limiting value.
• Fertilising an acidic pond before liming — phosphate is adsorbed by acid clay and the bloom never develops.
• Applying quicklime or hydrated lime to a stocked pond, causing a pH spike and mortality.
• Unit and dosage errors: forgetting that 1 ha × 1 m = 10,000 m³, or that 1 mg/L in that volume means 10 kg of chemical.
• Assuming more water exchange is always better — it imports pathogens, breaks biosecurity and violates discharge norms; aeration and bioremediation are usually the correct answer.

Related topics

Aquaculture systems · Fish diseases and health management · Shrimp farming · Fish biology · Limnology and pond fertility · Fish nutrition and feed