900 ish for mixed reef about 14-1500 gph for SPS tank. (That's just a general starting point ) Don't count skimmer just flow moving inside tank. Power heads that turn on and off are designed for it. Otherwise you'll get clicking noise as they cycle on and off. With two running full blast at 850gph each plus return it can still work just fine but depends on the corals and rock set up. You may have a hard time with some low flow loving corals. What is your plan for corals?