It really depends on how stocked your tank is. For a reasonably stocked or understocked tank, 25-30% per week as guppy said. For more heavily stocked tanks, I do 50% a week, and 50% twice a week for my Oscar cause he gets emotional when he wants a water change. For younger fish, I let the water sit aerated in an empty tank for a few days and siphon the water from there to the growout tank and add novaqua or any water conditioner. For larger older fish, i do it straight from the tap with the same temperature as the tank water.