• 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.

Temperature

• 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

pH

• 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  

• 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

Nitrite

• 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

Nitrate

• 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

Salinity

·         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)

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, 2^nd edition. British Small Animal Veterinary Association, 2001.