The Science of Water Changes Water changes help to remove unwanted and often toxic biological by-products from your aquarium water. Excess debris, fish waste, uneaten food, and decomposed plant matter turn into ammonia, then nitrite, then nitrates. These are bad for fish and also encourage algae blooms. Twenty-five percent water changes should be performed at least once a month in order to maintain a healthy aquatic environment.Gravel vacuums are designed to separate debris from gravel to "clean the gravel" as you perform your routine water changes. See:Learn About Aquarium Filtration |