numbers I gave you were considering over-filtration.. HTH
personally I would almost never put more then a 160gph filter on a 20 gallon tank.. that would be considering messy fish that wouldnt fit in the tank and river fish..
If your concerned about getting a emperor.. I have had one the 400 model.. It didnt do any better then my big wispier 5... and the GPH
is considerably lower on the Whisper 5 + no biowheel or spraybar.. experiance has made me see that the biowheel really isnt helpful you can get far more bio-filtration in an aquaclear with added bio-media..