Recommended distance for MH HQI

I am currently building a new canopy for my tank and I want to get my MH fixture higher over the tank, it is currently about six inches above the water surface. I need some recommendations about high high above the tank I can raise the fixture and still not affect the performance. I'm thinking the max height I would be willing to build my canopy to is about 16 inches, this would put the fixture about 12 inches above the waters surface. Is this acceptable or am I asking for problems.

The fixture is an outer orbit, with two 150 watt MH. The canopy I'm building will be on a 90 gallon reef tank.

 

I guess what he is asking is how deep is the tank?

 

Depends on the depth of the tank and the reflectors. Outer Orbits are pretty nice - you could make a 16" canopy and use the hanging kit and lower as needed, maybe 8-9 inches. If you build a smaller canopy you would have no room for adjustment though.

 

The tank would be roughly 25 inches deep with about a six inch deep sand bed and I will be using 2 supplmental 48 inch 110 watt super atinics