One other thought - the size of HOB is a big factor as well. For example, I have 2 aquaclear 110s on one of my 75s (with larger cichlids/plecos). That's around 1000 gallons of flow on a 75 gallon tank, or about 13x water turnover. I have some Whisper 60s on my 20g pleco tanks (300 gph on a 20g tank). A while back, when I tried an Aquaclear 20 (100gph) on a 29 gallon tank, it just wasn't keeping up with the waste output of the (somewhat heavily stocked) tank. So...if you can afford and have it fit on your tank, overfiltering is better, both in terms of size of HOB and size of sponge. That way you don't have to do a gravel vac every 2-3 days to suck out the piles of waste sitting here and there. Not that you should skimp on water changes, but it makes the tank look better in between WCs.