it depends on what your budget is. you can get a 24", assuming its a 30 gallon cube, 4 bulb t5ho fixture for a couple hundred bucks. it would be able to grow most of the corals you would want or you could go all out and get a radion xr15w g4 pro and spend around 500 bucks and get the same results. if you go the t5ho route just make sure you put good bulbs in it and you'll be fine.