I wouldn't be too concerned with the heat from one HQI on a tank that size. Just get a fan going over the sump. I've got 2 400s over my 200, and I'm currently running a 15 dollar WalMart fan over the tank for now. It's doing much better than I previously thought it would. 24" is fairly deep, but LEDs should be able to handle it, but with any LED it depends on the optics. Might be better off with the Chinese version LED on each side of the MH.