Pet Fish Doctor

4580 Crackersport Rd
Allentown, PA 18104


Water Quality and Pet Fish Health


  • 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
    • Ideal range > 6 mg/L


  • 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
    • Optimal level is 0 mg/l



  • 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
    • Maintain below 50 mg/l



·         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 Potential causes Historical & Clinical Findings Corrective measures
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
Ammonia toxicity 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 
Nitrate toxicity -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
Temperature 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 
Chlorine toxicity 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
pH -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


Suggested reading: 


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.