I agree with convict with the variable.mentioned above by Montel. I've maintained perfect water parameters with over filtration even on over stocked tanks. IE, 75 gallon tank with a fluVal fx5 rated to 928gph (~700gph with my custom plumbing and media) by the factory and a aquaclear 75.More than 50% will be needed, two 50% changes a week would be more in line