What kind of corals. If you want sps, I would say a 2-3 400 watt 20,000k MH would do it. If you just want softies you could go with VHO, PC, or a combo f either with some lower watt MH. The choices are endless. At 72" long (assuming standard 180g) I personaly would have 3 250w MH 10,000k bulbs with suplemenal VHO actinics. But I like the easy stuff, shrooms, xenia, frogspawn, brains, bubble coral, zoo's. BigMac is the resident MH expert maybe he will chime in here.