Water Quality Assessment in Aquatic Vet Medicine
World Small Animal Veterinary Association Congress Proceedings, 2018

With any animal, environmental conditions can affect their overall health, but with aquatic animals such as fish, proper water quality is an important part of a successful aquarium. Without clean water, the fish will be stressed and more susceptible to diseases and parasites. This lecture will provide veterinarians with information regarding how to test aquarium water and what the various water chemistry characteristics mean for the health of the fish. Correcting water quality problems is also included in the discussion.

Water quality can be measured with test kits available through pet stores or pond supply companies, or from many aquaculture suppliers. The simplest tests are small plastic strips with chemical pads attached that are dipped into the water to be tested. The pads change color which, when compared to a color chart, indicates the level of that substance in the water. These are fast, easy to use, inexpensive, and relatively accurate (they indicate a range rather than a precise measurement). Dry tablet tests are also available where a small tablet is dissolved into a test tube containing the water sample. Its color is then compared to a chart to determine the results. Some test kits have liquids that are mixed with the water to produce the color reactions. More expensive test kits use a spectrophotometer to electronically compare colors and these give more accurate results. Effective electronic meters are also available.


Ammonia (NH3) in the water reduces the ability of the fish to excrete nitrogenous wastes from their blood through the gills. As ammonia increases in the water, so do the waste products increase in the fish’s blood, causing toxicity, gill damage, and death. Ammonia is mostly converted to nontoxic ammonium (NH4+) at a pH level below 6.5, but above 6.5 ammonia can become toxic very quickly if allowed to accumulate. The higher the pH and temperature of the water, the more toxic ammonia becomes. The ammonia in the aquarium water is broken down by aerobic nitrifying bacteria into nitrite and then into nitrate. Properly operating biological filtration systems (after they have been cycled) should keep ammonia levels at 0.0 mg/L in the aquarium water.

In the event of a filtration system problem that creates high ammonia levels (>0.25 mg/L), ammonia neutralizing products can be used to bind the ammonia in a nontoxic form until water changes can be used to bring the ammonia level down. Failure to eliminate the ammonia through water changes will result in elevated nitrite levels a few days later.

Note: Some municipalities add chlorine or chloramine to the tap water to make the water safe for human consumption. Contact the local water service if unsure of the chemicals being used. Aquarists in areas that have chloramine added to the tap water need to use an ammonia remover as well as a chlorine remover to make the tap water safe for use in their aquarium.


Nitrite (NO2-) is produced by the aerobic bacterial nitrification of ammonia. It should also be maintained at a level of 0.0 mg/L. Nitrite is absorbed through the fish’s gills and causes methemoglobinemia, which reduces the ability of the fish’s blood to carry oxygen. Salt in the water at 0.1–0.3% salinity will block the absorption of nitrite by the fish’s gills. This is one of the reasons some aquarists add salt to aquariums with water changes. Remove any nitrite from the system by performing a partial water change.


Nitrate (NO3-) is produced by the aerobic bacterial nitrification of nitrite. While high nitrate levels are dangerous to saltwater fish and invertebrates, freshwater fish are very tolerant of high nitrate levels. Most freshwater fish can tolerate levels of 200 mg/L for short periods of time without significant problems. If nitrate levels exceed 20 mg/L, water changes can be used to lower the concentration. High levels of nitrate also promote algae growth.


The pH is the “potential of Hydrogen,” which is the measure of the hydrogen ions in a water (the acid/base balance). Most natural freshwater systems range from slightly less than 6.0 to about 8.5 in pH value. Most freshwater fish are highly adaptable to gradual changes within this range of water conditions. Rapid changes in pH are detrimental to fish, and it is very important that the aquarium has a stable pH. The stability of the pH is related to water alkalinity (buffering capacity).


Alkalinity is a measurement of the negative ions (e.g., hydroxide, carbonate, bicarbonate) in the water that buffer against pH shifts. Ideal alkalinity is in the 100–250 mg/L range for most freshwater fish species. Biological filtration in the aquarium uses carbonates, so over time the alkalinity level is reduced. As the alkalinity falls, the water in the aquarium may experience sudden, and deadly, pH shifts. To prevent this, increase the buffering capacity of the water to stabilize the pH. This can be done by adding carbonates to the water, or doing water changes with water that contains higher alkalinity.


Hardness is the measurement of metallic positive ions (e.g., calcium, magnesium) in the water. Water with high hardness usually also has a high alkalinity and pH. Hardness in aquatic systems is best at 100–250 mg/L, but some fish such as discus prefer softer water. African cichlids prefer hard water. Most fish will adapt to existing hardness as long as it is not too extreme of a change.

Chlorine and Chloramine

Chlorine and chloramine are used by water municipalities to make the water supply safe for human consumption. These compounds are extremely toxic to aquatic organisms and no amount can be tolerated in an aquarium. In cases of rapid fish loss in an aquarium after a water change, chlorine should be the first thing checked. Be certain to test for chlorine prior to treating the system with a dechlorinator. This will enable you to determine whether chlorine was the issue. There should never be any chlorine detectable in aquarium water! Add sodium thiosulfate to the system whenever chlorine is detected.


Freshwater tropical fish have a preferred optimum temperature range of 22–26 degrees Celsius (72–78 degrees Fahrenheit), but may be able to survive at temperatures about 5 degrees below or above this range. Gradual changes in water temperature within a fish’s optimum range seldom cause health problems. Ideally, water temperature fluctuations should be no more than 3°C change per day. Temperature shock can occur with rapid changes, especially from warmer water to cooler water. Increasing the water temperature will lower the saturation point of dissolved oxygen (warmer water holds less oxygen than cooler water). It will also increase the toxicity of dissolved substances such as ammonia, chlorine, and heavy metals.


Water testing is one of the most important aspects of aquarium maintenance. It is an important key in determining how well the filters are functioning. Keep a log of your water test results to monitor water quality changes over time. Always check the water quality when determining what may have caused fish loss. Water testing is not something to be taken lightly.

The following chart has general guidelines for the safe ranges and optimal levels of these parameters in freshwater aquaria and koi ponds.


Periodic partial water changes (25% of total volume) using dechlorinated tap water will keep aquarium water values normal. The frequency of changes will depend on the water test results, but normally once per week in new aquariums and 1–2 times per month in established aquariums is sufficient. Examples of incidents requiring increased water changes include toxin contamination, abnormal pH or alkalinity values, high ammonia, nitrite or nitrate levels, or over-medication. Test the water after performing a partial water change; if necessary, repeat partial water change to correct water quality parameters.


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