I needed more flow in my 55g and adding a modded MJ1200 was a tad too much for me, had to remove it and use another lower powerhead for the time being. The work fabulous but put out alot of flow. Determining how much flow you need though is not only by the size of tank, but more importantly, what kind of corals do you plan to keep? My tank is mostly LPS and softies/shrooms/zoo's, but also a few SPS, so I needed high flow for them, but not too much to rip apart my LPS. Iv'e got a 1500gph Seio and it's less flow than my "low flow" modded mj1200, wich supposedly does about 1500gph, more in my case obviously. And my return pump is probably about 500gph to the display. So, roughly 2000gph in a 55g, with mixed coral specimens and enough aquascaping and rockwork to break that current up and spread it around enough.