i have 2 emperor400's on my 55 gallon with a 12 inch oscar and 3 inch convict and i can go a week without nitrates going above 20-30 and i never get any amonia. i change the water every 20%. with enough filtration you can go longer without water changes. I have the magnum350 pro and an emperor400 on my 75 gallon with a 8 inch flowerhorn, 2 3 inch convicts, a 5 inch jack dempsey and some bottom feeders. i change the water in it every week too. it never has any problems either.