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
-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
-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
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
-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.