1" in your tank is roughly 20 gallons. So 20 gallons in 2 hours, is 10 gallons an hour, which is 240GPD. So thatd take it two days, without the rock displacement, plus that HUGE sump, lol. But Im sure its just a guestimate, as far as the 1" goes (could be more). So its close enough. Either way, it'll take no time in the long run. My "100 gpd" AquaFX Barracuda takes about 24 hours to fill up my 55g drum. Im not sure whats up with that, Im at 80 PSI IRRC.