I have a 55 gallon tank w/ 20 fish. Granted most of them are tetras. I have an emperor 400 on it now. It does do a good job keeping water clear but my nitrate levels even after water changes remain moderate. I have gotten advice from a few retailers as to what I should do. Some say the emperor is fine and others have suggested using a canister filter...Rena filstar xp3 to be exact. What do you advise?