How much water to change, and how often
Regular partial water changes are the single most effective thing you can do for a freshwater tank. They export the nitrate that biological filtration cannot remove and replace trace minerals the fish and plants use up. Enter your tank volume and roughly how heavily it is stocked, and this calculator suggests a weekly change: about 15 percent for a lightly stocked tank, 25 percent for a moderate one, and 40 percent for a heavy one.
Always match the new water's temperature to the tank and treat it with a dechlorinator before it goes in. These figures are a sensible starting routine; a nitrate test kit will tell you whether to change more or less for your specific tank.