Process by which the aquarium owner removes a portion of the water in the aquarium in order to remove a portion of the chemical contaminants in the aquarium.

It is generally known that a 15% water change every week is better than a 30% water change every two weeks because the water does not fluctuate as much if the water is changed every week than if it is changed every two weeks.

Partial water changes are done to maintain the water quality of aquariums. The amount removed must be limited in order to maintain an adequate amount of the helpful bacteria in the tank for maintaining the nitrogen cycle. Large water changes also make larger changes in the pH and/or temperature of the water more likely. Sudden changes in the quality of the water may be stressful to the fish and can cause disease and death.

Old water is removed while cleaning the gravel (if gravel is being used) with a gravel vacuum. The new water must be dechlorinated prior to being placed in the aquarium. Efforts are usually made to make the temperature of the water added identical to the temperature of the water in the tank. At times slightly cooler water is used to imitate a rainstorm. This induces breeding behavior in some fish. Larger than usual partial water changes may also be done to induce breeding behavior in somefish ... "Flash flood or rainy season, better breed quick!" said one fish to the other...

If the tank is a saltwater tank the saline ratio is also matched. At times chemicals are used to match the pH of the new water to the older water. If you dislike adding excessive chemicals to your tank and if your fish require a pH either more acid or more alkaline than your tap water then smaller and more frequent partial water changes are even more important.

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