they still produce waste, thats why you still have to keep on top of the water changes even if the tank apairs clear. but your most likely all set with that much of a water change. I do a simular 20% on wendsday and on sunday.Genin said:I'll do a 15-20% water change, wait two days, then do it again. So I change 40% of the water a week. I really don't need to do it that frequently because my guys eat everything and there's only 3 of them in there but I am an anal retentive dork.