- Change one-fifth to one-quarter of the tank's water every week for routine maintenance. This is also a good time to scrape off any excess algae or clean part of the tank's gravel. You should scrape the algae before the water change so the water siphon can more easily suck it up. One-fifth to one-quarter of the water is enough to remove any ammonia buildup from fish waste but not enough of a change to shock the fish.
- Test the tank water on a regular basis to be sure there isn't any sudden rise in ammonia, nitrates or chlorine. If there is, doing a partial water change similar to a routine maintenance water change often helps balance the water chemistry of the tank. According to "Freshwater Aquarium Problem Solver" (David E. Boruchowitz, 2006), a partial water change also helps clear up a bacterial bloom that can cause cloudy-white water.
- A general rule is that the larger the tank, the less often it needs water changes. But this depends on the amount of fish and live plants in the tank. The more fish and live plants in the tank, the more waste they will produce and so the water will need changing more often.
- Changing all of the aquarium water every week will not make the tank healthier. This will often make the tank water much unhealthier for the inhabitants. The tank needs to keep some bacteria, algae and old water in it in order to keep the water chemistry balanced.
- The dirtiest fish that requires the most water changes is the goldfish, according to "Goldfish: A Complete Pet Owner's Manual" (Marshall Ostrow, 1995). This is because the goldfish produces more waste than most other fish. This is also a reason that goldfish can live only with other goldfish, because most other species die from living in water that is so often fouled.
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