I've had best results with 1 250W per 2' of tank at 2' depth. If your tank has 2 center braces, 3 is the best solution. If you have only one center brace, then you might get by with 2 if you place them a little higher over the tank and stick with less demanding corals such as most LPS and softies. But for light hungry SPS, clams, and anemones - 3 would be much more ideal. JMTC & GL!