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Much depends on the size of the tank. The smaller the tank, the more water changes (in both amount and frequency) needed in order to maintain healthy conditions. Very small tanks often do best with 100% water changes. Larger tanks are more stable and, therefore, require less maintenance but still need some. It also depends on the stocking. One can have a heavily planted, low bioload tank that never needs water changes. The heavier the bioload, the more water changes become necessary.Dahly said:Some hobbyists advocate 50% changes everyweek.