- Water quality includes all physical, chemical and biological characteristics of water which regulate its suitability for maintaining fish.
- Poor water quality is the most common cause of morbidity and mortality in pet fish and the most common stressor that precipitates disease.
- Water quality should be monitored weekly and records should be maintained to monitor fluctuations.
- Water quality should be performed as part of the minimum database in every fish case.
- Reagents should be replaced yearly.
- Any fish may be affected regardless of age, sex and species.
- Each species has an optimal range for individual water quality parameters
- Poor husbandry practices such as overcrowding, overfeeding, inadequate water flow or filtration predispose to poor water quality.
- Acute or chronic stress resulting from exposure to poor water quality will often lead to reduced immune system function, predisposing fish to infection by opportunistic pathogens.
- Acute exposure to poor water quality can result in sudden and significant mortality.
- Chronic exposure to suboptimal water quality conditions can predispose fish to a variety of infectious diseases that ultimately lead to mortality.
- Fish are poikliothermic
- Ideal temperature varies with species. Freshwater tropicals prefer 75-80°F, marine tropicals prefer 78-84°F, koi and goldfish prefer 65-77°F.
- Chronic or rapid hypo/hyperthermia results in stress and immunosuppression
- Marine tropical fish are more sensitive to temperature changes then freshwater tropicals
- Ideal temperature changes are < 1°F/day
Dissolved oxygen (DO)
- Increases in water temperature and salinity decrease oxygen carrying capacity
- DO drops during the night due to respiration by animals and plants
- Expressed in mg/L or ppm
- Measure of the hydrogen ion concentration
- Logarithmic scale: change of 1 pH unit represents a tenfold difference in concentration.
- pH of 7.0 is neutral, pH < 7.0 is acidic, pH >7.0 is alkaline (basic)
- Ideal pH varies w/species. Most fish live between 5.5-8.5.
- freshwater aquariums do best with neutral pH
- marine aquariums 8-8.5
- More ammonia is present in the toxic form (NH3) at higher pH
- Slow changes in pH are best (0.3-0.5 units/day)
- Water with low alkalinity is more likely to undergo pH fluctuations
- Ammonia is the primary nitrogenous waste product of fish
- Nitrifying bacteria oxidize ammonia to nitrites and nitrites to nitrates
- New tanks/ponds that lack nitrifying bacteria will have increase in nitrogenous compounds (“New Tank Syndrome”) that resolves as the biofilter matures
- Damages gill tissue resulting in hyperplasia/hypertrophy and decreased O2 absorption.
- Two forms
- Ionized form (NH4+/ammonium)
- Non-ionized (NH3/ammonia) form is much more toxic
- Ammonia is more toxic in warm water, at higher pH and with decreasing salinity
- The temperature, pH and salinity can be used to calculate the actual amount of non-ionized ammonia present.
- Ammonia/chloramine binders can interfere with the Nessler reagent test
- High levels of nitrite and nitrate can interfere with the Salicylate method
- Most test kits report the total ammonia nitrogen in mg/L
- The only safe level for ammonia is 0 mg/L; the presence of any ammonia in the water is significant
- Ammonia is oxidized to nitrite (NO2-) by Nitrosomonas and other microbes
- Absorbed by the gills and oxidizes the haemoglobin (Hb) to methemoglobin (MetHb)
- Marine fish less sensitive due to higher levels of chloride in water
- Less toxic then ammonia but more then nitrate
- Reported in mg/L or ppm
- Nitrite is oxidized to nitrate (NO3) by Nitrobacter and other microbes
- Least toxic of nitrogenous compounds but eggs and fry are more sensitive then adult fish
- High levels indicative of infrequent water changes
- High levels stimulate algal blooms and decrease buffering capacity
- Reported in mg/L or ppm
· Measures the concentration of all dissolved salts in water
· Includes sodium chloride, calcium bicarbonate, calcium carbonate etc.
· Most commonly reported as parts per thousand (ppt), grams/liter, or as a percentage
o 1 ppt = 1 g/l= 0.1%
· Increases can be avoided by performing partial water changes rather then just topping off the pond/tank
· Ideal levels vary with species
o Marine fish require highest salinity (typically 30-35 ppt)
· Some plants are extremely sensitive to salt
· Maintaining fish at suboptimal salinity can result in osmoregulatory stress, impaired growth rates and reduced disease resistance
Hardness and Alkalinity
· Hardness represents the concentration of polyvalent mineral cations in the water including Calcium and Magnesium.
o Expressed as ppm (mg/L) of calcium carbonate. This can be measured with the GH test kits.
· Alkalinity is a measure of water’s buffering capacity (measures the mineral anions). Anions include bicarbonates, carbonates, and hydroxides.
o Total alkalinity is expressed as ppm (mg/L) calcium carbonate. This is sometimes referred to as KH or Carbonate hardness.
· Since calcium carbonate is the single largest source of these ions, the alkalinity and hardness values as mg/L or ppm will usually be similar.
o Water softener will result in low GH but not affect KH.
o KH can be higher then GH when sodium bicarbonate is added.
· Hardness, alkalinity and pH are closely related. Soft water is usually acidic, while hard water usually has a basic pH
· Soft water (0-75 ppm), Moderately hard (75-150 ppm), Hard (150-300 ppm), Very hard (>300 ppm)
|Water quality condition
|Historical & Clinical Findings
|Hypoxia – low DO
|Overcrowding, poor water flow, inadequate aeration, algae die-off, filtration/system failure, increase temperature, chemicals (formalin)
|-Acute – high mortality, increased opercular rate, pale gills, piping (gasping at surface), gathering in well aerated areas-Chronic – lethargy, anorexia, poor growth, opportunistic infections
|-Aerate aggressively, monitor ammonia/nitrites, evaluate system and filtration, decrease stocking density-In emergency, hydrogen peroxide (3%) can be added at a rate of 0.5 mls/l
|Overcrowding, overfeeding, build-up of organic debris, infrequent water changes, inadequate biological filtration as seen in “New Tank Syndrome” due to lack of nitrifying bacteria
|Mortality, neurologic/behavioral abnormalities, lethargy, anorexia, poor growth, secondary infections, injected fins, gill hyperplasia and hypertrophy
|Reduce or eliminate feeding, decrease stocking density, 25-50% water changes, evaluate and maintain pH (avoid alkaline pH), maintain good oxygenation, ammonia binders, evaluate biofiltration, low doses of salt increases the ionization of ammonia
|Nitrite toxicity Brown blood disease methemoglobinemia
|-See ammonia toxicity.-Nitrite oxidizes Hb MetHb resulting in hypoxia
|-Respiratory signs – increase opercular rate, piping (gasping at surface), gathering in well aerated areas, death-Gills and blood may show brown discoloration due to MetHb
|-oxygenation-Salt to 0.12%; chloride ions compete with nitrite ions for absorption -See ammonia toxicity for other treatments
|-See ammonia toxicity -most common cause is infrequent water changes
|Poor growth, lethargy, anorexia, poor growth, opportunistic infections, injected fins
|-Water changes, remove organic debris-aquatic plants may remove nitrates from water
|Rapid temperature fluctuations can result in temperature shock. -Temperature changes can result from equipment malfunction and weather changes
|-Hypothermia: inactive, lying on bottom, lethargy, anorexia, death
-Hyperthermia: restlessness, sudden death
|-Temperature correction-Fluctuations greater then 1°C/hour may cause temperature shock; however, in life threatening emergencies, rapid temperature changes may be required
|Failure to dechlorinate water
|Respiratory signs, sudden death
|-Dechlorinators such as sodium thiosulfate (3.5 mg/L)-Aeration of water for 24 hrs in open topped container will dissipate chlorine-oxygenate water
|Gas supersaturation, Gas bubble disease
|Supersaturation of water caused by faulty equipment, sudden elevations in temperature, venturi effect
|-Gas emboli formed in circulation and tissues. Gas bubbles may be seen in eyes, on fins, gills and under skin, behavioral abnormalities, positive buoyancy (small fish), death-Holding a light source close to the fish can help visualize emboli.
|-Elimination of excess gas from water-Repair faulty equipment
|Hydrogen sulfide toxicity
|-H2S is produced under anaerobic conditions at the bottom of ponds/aquaria or in filter beds that are not completely aerated. -Disturbing the bottom can release into water column
|-Lethargy, anorexia, piping, sudden death -Characteristic rotten egg odor
|-Aggressive aeration, water changes, remove decomposing detritus-Maintain aerobic conditions in tank/pond/filter-Potassium permanganate at 2 mg/L can oxidize/detoxify hydrogen sulfide
|-Rapid pH fluctuations are most problematic.-pH fluctuations most common in systems with low buffering capacity (alkalinity). -pH can increase during algal blooms and in heavily planted ponds/aquaria-Build up of organic debris can decrease pH
|Lethargy, stress, skin lesions, behavioral changes, corneal edema, gill irritation with increase mucus production, respiratory signs, death
|-Many commercial preparations/buffering compounds available for adjusting pH, sodium bicarbonate (improves alkalinity)-water changes-limestone or crushed oyster shell can be used to increase alkalinity/pH
Boyd CE. Water Quality An Introduction. Kluwer Academic Publishers. 2000.
Noga EJ. Fish Disease: Diagnosis and Treatment. St. Louis, St. Mosby, 1996.
Wildgoose, WH. BSAVA Manual of Ornamental Fish, 2nd edition. British Small Animal Veterinary Association, 2001.