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Aquatic Veterinary Medicine – Water Quality Comprehensive Notes

Learning Objectives

  • Identify potential contaminants and their consequences (chlorine, chloramines, heavy metals, microorganisms, toxins, excess nutrients, algal bloom, agro-chemicals, antibiotics, plastics).
  • Explain significance of key physicochemical & microbiological parameters for fish physiology (temperature, pH, dissolved oxygen, ammonia, nitrite, nitrate, alkalinity, hardness, salinity, turbidity/clarity, CO₂).
  • Select appropriate tools, sampling sites, frequencies and analytic techniques (titrimetric, colorimetric, photometric, electronic probes, data loggers).
  • Describe the full nitrogen cycle and the role, maturation and protection of the biofilter in closed/recirculating systems.
  • Predict how variations in water quality drive stress, immune suppression, reduced performance, disease expression and treatment efficacy.

Why Water Quality Matters

  • Fish and aquatic organisms inhabit – and breathe – their waste: “They live in their toilet bowl”.
  • Poor water quality = chronic or acute stress ⇒ ↓ immune competence, ↓ feed conversion & growth, ↓ fecundity, gill impairment, ↑ secondary pathogens, mass mortalities.
  • Sudden or wide fluctuations exacerbate stress, especially at high stocking densities.
  • Environmental & public-health impacts: toxic algal blooms, human zoonoses, contamination of downstream waters.
  • Water chemistry modifies pharmacokinetics & treatment success – all parameters must be measured, managed, mitigated.

System & Operational Variables

  • Species held (coldwater vs tropical, freshwater vs marine vs brackish, stenohaline vs euryhaline).
  • Water source: municipal (tap), well, surface (river/lake), open sea, bay, synthetic sea-salt mixes.
  • Production / holding system type: fishbowl, ornamental pond, pet store tanks, public aquaria, hatchery, nursery, RAS, flow-through, ocean cage, research facility, aquaponics.
  • Stocking density, biomass loading, feed rate & formulation determine waste production (faeces, uneaten food ⇒ ammonia, CO₂, organic load).
  • Volume on hand for exchanges / emergencies; available equipment for filtration, aeration, heating/cooling, monitoring.

General Testing Recommendations

  • Test strips: quickest but least reliable – avoid diagnostic decisions.
  • Liquid colorimetric kits (visual or photometer): accurate, inexpensive; check reagent expiry, follow instructions.
  • Electronic probes / multi-parameter meters: rapid, loggable, precise; require routine calibration & maintenance; higher upfront cost.
  • Reference ranges are species & life-stage specific – establish benchmarks before sampling.

Temperature (^{\circ}\mathrm{C}/^{\circ}\mathrm{F})

  • Fish are ectotherms: metabolic rate roughly doubles / halves with every 8^{\circ}C change (Q10 ≈ 2).
  • Triggers spawning, digestion, immunity, development; rapid changes → stress.
  • Vertical stratification (thermocline) in ponds/lakes; turnover events can oxygenate or catastrophically de-oxygenate deeper layers.
  • High temperatures accelerate:
    • Oxygen depletion (↓ gas solubility).
    • Ammonia toxicity (shift toward unionised NH₃).
    • Microbial growth & disease onset.
  • Coral bleaching: sustained marine heatwaves force corals to expel zooxanthellae.
  • Measurement: glass/alcohol thermometers, digital handhelds, dataloggers, IR guns (surface only). Take multiple depths/locations; graph min-max trends.
  • Transport & acclimation: match shipping water to destination temperature gradually to avoid thermal shock.

Dissolved Oxygen (DO) & Oxygen Saturation

  • DO = mg/L or ppm of free O_2 in water. Saturation = % of maximum possible at given T, salinity (SAL) & atmospheric pressure.
  • Ideal thresholds:
    • General min: \ge 5\,\mathrm{mg\,L^{-1}} AND 70{-}100\% saturation.
    • Intensive production: 5{-}12\,\mathrm{mg\,L^{-1}}.
    • Ornamental systems: >10\,\mathrm{mg\,L^{-1}}.
  • Daily photoperiod pattern with algae/plants: peak late afternoon, nadir at dawn.
  • Supersaturation (>100 %) ⇒ gas bubble disease (microbubbles in eyes, gills, skin); avoid sudden T or pressure drops.
  • Low DO clinical signs:
    • Acute: surface gulping, pipe-hanging, schooling near inflow.
    • Chronic: poor growth, poor reproduction, parasite outbreaks.
  • Cold-water species (e.g., salmonids) more susceptible because of higher O₂ demand at low temperatures.
  • Sampling: avoid aeration outlets; titrimetric Winkler or probe; store/maintain equipment per manufacturer.

pH

  • Definition pH = -\log_{10}[H^{+}]. One-unit change = 10× change in [H^{+}].
  • Natural waters typically 6.0 – 8.5.
  • Directly modifies toxicity of ammonia (NH₃ rises with pH) & hydrogen sulfide; affects solubility of metals.
  • Freshwater often has lower buffering capacity ⇒ larger diurnal swings vs seawater.
  • Tolerance ranges (approx.):
    • Freshwater fish: 6.0 – 9.0 (with gradual change).
    • Marine fish/inverts: 7.8 – 8.4.
  • Ocean Acidification: CO2 + H2O \leftrightarrow H2CO3 \leftrightarrow H^{+}+HCO3^{-} ⇒ loss of CO3^{2-} hampers calcification.
  • Clinical signs of pH stress: lethargy, excess mucus, skin/gill lesions, sudden death, secondary infections.
  • Instruments: calibrated pH-meter, portable probe, accurate test kit; routine trend analysis.

Nitrogen Cycle & Biofiltration

  1. Protein catabolism & decaying organics release ammonia NH3/NH4^{+}.
  2. Nitrosomonas/Nitrosospira oxidise ammonia → nitrite NO_2^{-} (acid-producing).
  3. Nitrobacter/Nitrospira oxidise nitrite → nitrate NO_3^{-}.
  4. Plants/denitrifiers reduce nitrate to N_2 gas.

Biofilter facts

  • Houses nitrifiers on high-surface-area media; essential in RAS, ponds, aquaria, aquaponics.
  • Cycling/maturation: 2 – 6 weeks; requires O2 and alkalinity (bicarbonate as inorganic C source) \Rightarrow monitor KH>100\,\mathrm{mg\,L^{-1}\ as\ CaCO3}.
  • Optimal temperature: 28{-}36^{\circ}C; but active 12{-}58^{\circ}C.
  • Sensitive to antibiotics, chlorine, heavy metals; protect during treatments.
  • Start-up strategies: low stocking/feeding; fish-less ammonia dosing (NH₄Cl/NH₄OH); commercial bacterial inocula.

Ammonia

  • Two species: unionised NH3 (FAN/UIA, highly toxic) and ionised NH4^{+}.
    NH3 + H2O \leftrightarrow NH_4^{+} + OH^{-}.
  • Conversion depends on pH & T; calculate UIA from TAN, pH, T tables.
  • Guideline limits: \text{TAN} \le 0.5\,\mathrm{mg\,L^{-1}}; UIA \le 0.1\,\mathrm{mg\,L^{-1}}.
  • Clinical: lethargy, excess mucus, haemorrhagic gills ("burns"), neuro signs, mortality, fishy smell.

Nitrite

  • Causes methaemoglobinaemia (“brown-blood disease”).
  • Ideal: 0 ppm; should remain <2 ppm.
  • Clinical: rapid opercular beats, brown gills, hypoxia signs.

Nitrate

  • Relatively non-toxic but chronic >20–50 ppm reduces growth, appetite, survival (higher tolerance e.g. tilapia).
  • Removal: water changes (25–70 %), denitrifying reactors, phyto-remediation.

Sampling & Frequency

  • Established systems: weekly; immediately after water changes or clinical signs.
  • Cycling/new systems: daily until TAN/NO₂ fall to target.
  • Preserve by immediate analysis or filter (0.45 µm) and freeze.

Emergency Management

  • High NH₃: reduce feeding, large water change with dechlorinated water, lower T & pH if species allows, add binders (zeolite, commercial detoxifiers), seed biofilter.
  • High NO₂: do NOT acidify (↑ toxicity); add chloride (NaCl) to competitively inhibit uptake, large water change, improve biofiltration.

Alkalinity (KH) & Hardness (GH)

  • KH = bicarbonate/carbonate alkalinity (mg/L as CaCO_3). Buffers pH; consumed by nitrification.
  • GH = divalent cations (Ca²⁺, Mg²⁺) – skeletal/ shell formation, osmoregulation.
  • Classification (approx.):
    • Soft: <70 ppm GH
    • Moderate: 70–140 ppm
    • Hard: 140–320 ppm
  • Target KH >100 ppm for stable pH & biofilter.
  • Adjustments:
    • ↑ KH: agricultural lime, hydrated lime, crushed coral/shell, baking soda.
    • ↓ KH/GH: dilution with RO, distilled, or rainwater.
    • ↑ GH: calcium chloride, magnesium sulfate (Epsom).
  • Field titrimetric kits 50–500 ppm; verify reagent expiry.
  • Deficiency signs: flexible bones, malformed fry, pH crashes (“old tank syndrome”).

Old Tank/Pond Syndrome

  • Chronic neglect → depleted KH, low pH (<6), compromised nitrifiers, rising TAN masked by acidity.
  • Management: gradual water & buffer additions (5–10 % changes) to avoid sudden spike in toxic NH₃.

Salinity & Total Dissolved Solids (TDS)

  • Salinity measured in parts-per-thousand (ppt): FW <2 ppt; brackish 2–16 ppt; seawater ~35 ppt.
  • Essential ions: Cl^{-}, SO4^{2-}, HCO3^{-}, Br^{-}, Na^{+}, Mg^{2+}, Ca^{2+}, K^{+}, Sr^{2+}.
  • Evaporation ↑ salinity; top-up with freshwater rather than seawater to maintain target.
  • Osmoregulatory window species-specific; deviation costs energy, reduces growth/health.
  • Therapeutic uses in FW fish: 0.3–5 g/L (support mucosa, osmoregulation); 20–30 g/L, 5-min baths for ectoparasites in tolerant species.
  • Instruments: handheld refractometer, conductivity/TDS probes; calibrate and temperature-compensate.
  • TDS = all dissolved ions (<2 µm) + organics & some metals – correlated but not equal to salinity.

Water Clarity / Turbidity

  • Suspended/dissolved particles scatter light; sources = clay, silt, organic detritus, algae, microbes, coloured humics.
  • Species adapted to turbid environments (catfish, walleye) become stressed in overly clear water.
  • Excess turbidity ↓ light penetration ⇒ affects photosynthesis, food webs, smothers eggs, abrades gills.

Common Water Contaminants

  1. Chlorine & Chloramines

    • Municipal disinfection 0.5–8 ppm; fish toxic >0.03 ppm.
    • Neutralisation: aeration (~24 h, not effective for chloramine), sodium thiosulfate 7.4\,\mathrm{mg\,L^{-1}} per 1 ppm Cl₂, activated carbon, commercial conditioners.
    • Chloramine-T (Halamid®) used therapeutically (ectoparasites) when dosed per label.
    • Excess ⇒ gill mucous, dysfunction.

  2. Excess Nutrients & Harmful Algal Blooms (HAB)

    • Inputs: fertiliser runoff (N,P), iron, detergents, animal waste, aquaculture effluent.
    • Effects: eutrophication, DO crashes, toxins (cyanotoxins, saxitoxin, brevetoxin), gill clogging, light shading.
    • Human/animal health risk; monitor advisories (EPA, NOAA).

  3. Heavy Metals

    • Sources: industrial discharge, mining, galvanised pipes, urban runoff.
    • Bioaccumulate (liver, kidney, gills) & biomagnify up food chain (e.g., Hg, Pb).
    • Organ damage: hepatic, renal, neuro, immune, reproductive.
    • Copper: therapeutic for ectoparasites but toxic to crustaceans/corals; leaches from old plumbing. Remove via ion exchange, EDTA chelation, water changes.
    • Zinc: toxicity ↑ in low pH, soft water.

  4. Agro-chemicals & Antibiotics

    • Pesticides, herbicides, fungicides (e.g., glyphosate) can bioaccumulate; endocrine & developmental effects.
    • Antibiotic residues from veterinary, hospital, sewage promote resistance, alter microbiota.

  5. Plastics / Microplastics

    • Physical & chemical hazards; vector for pathogens; still under investigation.

  6. Biological Hazards (Pathogens)

    • Opportunistic bacteria (e.g., Klebsiella pneumoniae outbreak in tilapia), parasites introduced via water or carriers.

Water Quality Planning & Record Keeping

  • Design systems with adequate mechanical & biological filtration, flow, and aeration for biomass & feed load.
  • Maintenance:
    • Clean mechanical filters routinely; do NOT sterilise biofilter media.
    • Vacuum substrates, remove detritus to prevent anaerobic pockets / H_2S formation (rotten-egg smell).
    • Schedule preventive tasks through checklists; log water parameters, equipment calibrations, treatments.
  • Use buffers, conditioners, probiotics prudently; confirm compatibility with resident species & biofilter.
  • Emergency backup: reserve water volume, power, aeration, oxygen cylinders.

Ethical, Environmental & Practical Implications

  • Veterinarians & producers have duty to safeguard animal welfare by maintaining optimal water quality.
  • Preventing contaminant release, nutrient runoff and HABs protects surrounding ecosystems and public health.
  • Responsible chemical use (e.g., copper, antibiotics) minimises resistance, residues and non-target impacts.
  • Data-driven management (trend analysis, minimum-maximum alarms) enables early intervention, reducing catastrophic losses and improving economic sustainability.

Equations & Reference Values (Quick Sheet)

  • pH = -\log_{10}[H^{+}]
  • Unionised ammonia fraction (UIA) derived from \mathrm{TAN},\;pH,\;T tables.
  • DO solubility falls ~1 mg/L per 12^{\circ}C rise (approx.).
  • Chlorine neutralisation: \mathrm{Na2S2O3}\;7.4\,\mathrm{mg\,L^{-1}} per 1\,\mathrm{mg\,L^{-1}} Cl2.
  • KH, GH, TAN, NO₂, NO₃ ideal targets (generic):
    • KH \ge 100\,\mathrm{mg\,L^{-1}\ as\ CaCO_3}
    • GH \ge 50\,\mathrm{mg\,L^{-1}\ as\ CaCO_3} (species-dependent)
    • TAN \le 0.5\,\mathrm{mg\,L^{-1}}; UIA \le 0.1\,\mathrm{mg\,L^{-1}}
    • NO_2^{-} < 2\,\mathrm{mg\,L^{-1}} (0 ideal)
    • NO_3^{-} < 20{-}50\,\mathrm{mg\,L^{-1}} (higher for tolerant species)

Study Tips & Connections

  • Relate each parameter to fish physiology systems (respiratory, osmoregulatory, metabolic, immune) for integrative understanding.
  • Compare terrestrial veterinary concepts (e.g., acid-base balance, respiratory gas exchange) with aquatic equivalents (pH/KH, DO).
  • Use case studies (heatwave coral bleaching, old tank syndrome, HAB events) to contextualise numerical limits.
  • Practice calculations: UIA from TAN; chlorine neutralisation dose; conversion mg/L ↔ ppm (1:1 in water); % saturation from mg/L & solubility tables.
  • Remember that water chemistry seldom changes in isolation – anticipate cascading effects (e.g., ↑T ⇒ ↓DO, ↑NH₃ toxicity, ↑microbial growth).