Testing water quality in a goldfish aquarium or pond is an essential element of routine maintenance. It provides advanced warning of problems, reassurance that problems are less likely, and elimination of possibilities when diagnosing a problem.

There are 4 main aspects of water quality to test, all related to each other chemically.

Ammonia and Nitrite

Ammonia and nitrite levels can build up to dangerous or even fatal levels in a new tank. Ammonia is present in goldfish faeces, and is produced by rotting plant material and uneaten food. Certain types of bacteria feed on ammonia, turning it into nitrite, then nitrates. Until these bacteria are present in sufficient numbers, the ammonia and nitrite levels will rise.

The ideal levels of ammonia and nitrite are zero, but small amounts (less than 4 parts per million) can be tolerable. However, in a new tank with no ammonia-feeding bacteria the ammonia level will peak after a week, with the nitrite levels peaking a fortnight later. It is very important to have chemicals to deal with these toxins at hand during the first month of the aquarium.

Nitrate

As mentioned earlier, nitrites are eventually converted to nitrates, which are much less toxic to goldfish. Nitrates are still dangerous, and although goldfish that have been in the aquarium for a long time can build up tolerance, new goldfish will struggle to survive in water with high nitrate levels.

Regular, adequate water changes should be enough to prevent nitrates building up to dangerous levels. If testing shows high nitrate levels, either increase the frequency or the volume of water changes.

pH Level

pH is a measure of acidity and alkalinity. The scale runs from 0 (most acidic) to 14 (most alkaline) with 7 being neutral.

The optimum pH for goldfish water is slightly alkaline, but goldfish are happy with anything between 6 and 8. The most important thing to remember about pH management is that changes in pH are more harmful to goldfish that being outside the optimum range. Stable pH is the key.

Water Hardness

Water hardness is important because it regulates the pH of the water by a process known in chemistry as buffering. In practice it means substances dissolved in the aquarium water absorb the impact of changes that would otherwise change the pH of the water.

There are two measures of water hardness: general/total hardness and carbonate hardness. It is only necessary to measure one or the other, not both.

General/total hardness is often expressed as dH, GH or dGH. It should be at least 4dH to ensure buffering works, and goldfish can tolerate levels up to 20dH.

Carbonate hardness is expressed as mg/L and should be between 70mg/L and 400mg/L.

Testing Routine

Testing for ammonia and nitrites should - in theory - be unnecessary in an established goldfish aquarium. Testing for nitrates should be done with every water change, i.e. weekly. The same goes for pH and hardness tests.

The other time to perform testing is when something appears to be wrong: goldfish not eating, one dying, new fish dying and upon discovery of illness or disease.

It is also very important to record the results of testing. If there is an outbreak of chilodonella for example - which can be caused by poor water quality - it is no good knowing what the pH of the aquarium water is on the day of the outbreak if you can’t remember what it was in the days and weeks running up to the outbreak.

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