Well, I ran a whisper 3 on a 29 gallon, with goldfish, for years and it did ok. But I also had an undergravel filter in there, with a power head to pull the water thru the gravel. That combination was very succesfull for me.
Just remember to not throw out the cartridges as often as the manufacturer tells you to do. Just rinse and squeeze them out in old tank water during a water change, and then put them back in. When they get to the point where they are falling apart, just replace one at a time. Not both at once.
Even better, stuff the new one into the gap between the old cartridge and the wall of the filter closest to the tank for a few weeks, and then replace the old with the new. That way, water will flow thru the old and carry some of the beneficial bacteria into the new.
