Best distance between Metal Halide and Water Surface

[lpd154]

I'm in the process of building a new canopy for my tank and I am trying to get advise on the placement of my metal halide fixture. I'm currently using an outer orbit fixture that has two 150 watt metal halides. Using the factory supplied brackets the distance between the light fixture and the water surface is about 6 inches. I would like to mount this fixture at the top of the canopy but before I do, I would like to find out as much as possible about the proper distance to water separation of metal halides.

My current tank is a common 55 gallon rectangle, but since I'm on the verge of purchasing a 90 gallon tank I am going to build the canopy to fit the 90 gallon tank.

 

[flower]

I would think the 6 inches is the minimum. The higher the light the less penetration of light on the bottom of the tank. I have a 90g and use a retrofit 2 X 175w MH with actinic…All coral, even on the bottom is very happy and my lights are about 10 inches away from the surface.
If I were handy I would make the canopy about 12 inches with doors, so I could access my tank to maintain it without removing the whole hood, which is a real pain in the butt.

 

[zman1]

If this is what you have, the manual states - DO NOT place fixture underneath a canopy or hood. This light fixture is designed to be placed above an aquarium. It is not intended to be placed under a canopy. The light fixture is to be used over completely covered aquariums only.
http://www.current-usa.com/files/Out..._1070-1074.pdf
If it's not the model you have, check the manual that came with yours. Any other advice would be irresponsible… It's your tank, your decision.

 

[flower]

The manual is talking about the standard canopy. I removed the top of mine and put my lights above it, not under it.
A handmade canopy 12 inches with a door and enough ventilation to let heat escape should be no problem.

 

[acrylic51]

I'd go no less than 12"