Aquarium Water Changes

Why partial water changes matter, how much and how often to change, dechlorination, temperature matching, and gravel vacuuming.

Why water changes matter

Partial water changes dilute accumulated nitrate and other dissolved waste and replenish minerals. The main reason most aquariums benefit from routine water changes is to keep nitrate from building up over time, since nitrification produces nitrate continuously.

How much and how often

Volume and frequency depend on stocking and feeding. A roughly 30% change is a reasonable general figure. Heavily stocked tanks may need more than 50% every two weeks; moderately stocked tanks far less often; lightly stocked tanks less again. A common practical rule is to change water when nitrate climbs over about 80 ppm.

Dechlorination

Most tap water is treated with chlorine or chloramine, both toxic to fish and to nitrifying bacteria. Untreated tap water must be neutralised with a dechlorinator (water conditioner) before or as it enters the tank, unless the supply is verified to contain no chlorine or chloramine.

Temperature matching

Replacement water should be close to the tank temperature to avoid thermal shock to fish. Matching temperature is especially important for larger changes and for sensitive species.

Gravel vacuuming

Using the water change to vacuum detritus from the substrate removes trapped organic waste before it decays into ammonia. An exception is an undergravel filter, where deep vacuuming removes biofiltration capacity and can trigger ammonia spikes.

Common mistakes

Adding cold or hot unconditioned water, skipping dechlorinator, or doing very large sudden changes on a neglected tank can stress or harm fish. Consistent moderate changes are safer than rare massive ones.