Performing Water Tests: What Do They Tell Us?
World Small Animal Veterinary Association Congress Proceedings, 2019
N. Saint-Erne1; J. Tepper2
1Certified Aquatic Veterinarian, WAVMA, Glendale, AZ, USA; 2Long Island Fish Hospital, Manorville, NY, USA

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 keeping them healthy. 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 pond 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 for some water tests.


Koi (Cyprinus carpio) have a preferred optimum temperature range of 18–25 degrees Celsius (65–77 degrees Fahrenheit), but are able to survive at temperatures 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.

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 by fish. There should never be any chlorine detectable in aquarium or pond water! Add sodium thiosulfate or other dechlorinator to the koi pond whenever adding tap water if chlorine is detected.


Ammonia 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 nitrogenous waste products increase in the fish’s blood, causing toxicity, gill damage, and death. Ammonia is mostly converted to nontoxic ammonium 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 pond 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 water.

In the event of a filtration system problem that creates high ammonia levels (>0.25 mg/L), Ammonia neutralizing products can be added to the pond 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 chloramine to the water to make the tap water safe for human consumption. Contact the local water service if unsure of the chemicals being used in the tap water. Fish keepers in areas that have chloramine added to the tap water need to use an ammonia neutralizer as well as a chlorine remover to make the tap water safe for use in their aquarium or pond.


Nitrite is produced by the aerobic bacterial nitrification of ammonia. It should also be maintained at a level of 0.0 mg/L. Nitrite 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. Remove any nitrite from the system by performing a partial water change. Nitrite will also be converted to nitrate by a different species of aerobic nitrifying bacteria.


Nitrate 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 100 mg/L for short periods of time without significant problems. It is preferable to maintain nitrate below 10–20 mg/L, and periodic water changes in the pond should keep the nitrate level down. If nitrate levels exceed 20 mg/L, additional water changes can be used to lower the concentration. High levels of nitrate also promote algae growth.


The potentia hydrogenii, or power of hydrogen, is the acid-base balance in water. Most freshwater fish are highly adaptable to slow changes in the pH as long as it is not too extreme (less than 5.5 or above 8.5). Rapid changes in pH are more detrimental to fish, and it is very important that the pond water has a stable pH. The stability of the pH is related to water alkalinity and hardness. If there are extremes of pH, or rapid fluctuations, it is likely because the alkalinity is too low.


Alkalinity is a measurement of the negative ions (e.g., hydroxide, carbonate, bicarbonate) in the water that buffer against pH shifts. Ideal alkalinity for koi is in the 100–250 mg/L range. As the alkalinity falls, the water in a pond may experience sudden, and deadly, pH shifts. If it happens in your system you can increase the buffering capacity of the water to stabilize the low pH by adding supplements such as sodium bicarbonate or calcium carbonate to raise the 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 pH. Softening the water will lower the mineral content and the pH. Hardness in koi ponds is best at 100–250 mg/L. Most fish will adapt to existing hardness as long as it is not too extreme of a change.


Water testing is one of the most important aspects of maintenance for your filtration systems. It is an important key in determining how the biological filters are functioning. Keep a logbook of the water test results, to monitor changes in the water parameters. Water testing is not something to be taken lightly.

Periodic partial water changes using dechlorinated tap water will keep pond water values normal. The frequency of changes will depend on the water test results, but normally once per month in established ponds 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. Test the source water (tap water) to ensure it has the correct water parameters for the fish, and adjust with chemicals as necessary.


Speaker Information
(click the speaker's name to view other papers and abstracts submitted by this speaker)

N. Saint-Erne
Certified Aquatic Veterinarian
Glendale, AZ, USA

J. Tepper
Long Island Fish Hospital
Manorville, NY, USA

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