I am running twin 250W HQI's over a 24" tall tank it is does great. The heat issue will be a real problem with the 400W bulbs. If you don't mind lots of fans running and a chiller running a lot more of the time, go with the 400W bulbs, but in my mind, it is more than you need. My tank grows Acros no problem with the 250W. I have not once regretted the installation choice of the 250's over the 4'x2'x2' tank. More light is not what I need. Better water quality is what it is all about to have a nice, healthy, and sustainable SPS tank.