Usually you should look for a filter that will turn over the tank 4 times an hour, so 60x4 = 240gph. It's also best to go a bit higher than that as when a filter says 240gph, it can mean without filter media, and certainly it doesnt do 240gph when dirty.
It's best to overfilter, if you get a filter that does say 300gph you should be able to adjust the flow rate on it anyway. Certainly get a filter that does more than 240gph. If you're getting messy fish then you'll want to overfilter even more.