I have a koralia 2 and 4 in my 30 gallon bowfront. The K4 seems like a lot, so I turn it on and off depending on the time of day (auto-timer), and the K2 is always on. K2 - 600gph, K4 - 1200gph. I have 2 backup powerheads, a 106gph and a 400gph that used to be in the tank.