Well I have a spray bar system on the bottom of the tank with eggcrate on top of that and have rock rubble on top of that so I'm guessing from the rock to the lights is maybe 18". I constructed a frame on the back wall of the tank allowing me to run the rock work vertically up the back and still have about an inch of open area back there. Was thinking something similar to a cave, I have a couple of more rocks kuring that are going to be attached to the top frame. Decided on T5 for heat purposes and power requirements. Plus the added benefit of not having to purchase a whole setup all at once.