How high should a lighting fixture be above a tank?

I just got a lighting fixture for my 75g reef. It has 2 250w MHs and 2 54w PC Actinics. The tank is 21" deep and is open. I currently only have soft corals, fish and inverts in the tank. The LR goes up to about 4" below the water line. I want to be able to put hard corals and an anemone in the tank later.

I would like to know how high above the tank I should suspend the lighting fixture to give the best light, but not cook the tank.

I would appreciate you input.

Thank you,

 

There's no absolute answer as every tank is different. Just put them 7-10 inches above the water to start. If your water gets too hot in the summer or it seems too much light for the corals then raise it a bit. Or if you go SPS mostly then try to lower it.

 

I am going to say 12-15".