Metal halide: How far above the water?

I will have a 150w current pendulum HQI over a 75g mixed reef. How high above the water should I have it? I would like to have it in a canopy, which is the main reason I ask. I will have a 1/4 hp chiller & (2x) 4" pc fans(1 on each end) & an open back to the canopy.
Edit: I will be adding retrofit HOT5's in the future.

Tank dimensions: 48.5" Long 21.5" High 18 7/8" Wide.

Edit: 2 air exchange vents on the top. Heat rises..

 

1.243 centimeters exactly, If its to high your tank won't get any light. If its to close the water will boil. Now this is important if the surface of your water ripples your halide must reflect those ripples or else you will get areas of spontaneous evaporation that will vibrate the molecules in your tank weakening the glass and killing the fish. As you may have guessed by now the above part of my answer is completely fake, I usually do about a 10 inches, but its really about you being able to access the tank with out burning yourself on the light.

 

heh. I was reading your response going... 'What?' Aye, 8-10 inches usually works great for me too and should work great on an 18 inch deep tank. I personally had to go higher on my current tank but it is only 12 inches deep with a 150w halide.

 

lol. I hadn't thought about me being hurt by the light. My canopy design is 8" high. Guess I should change this to make it a foot?

Better yet, could I keep my canopy high, but have a cutout for the fixture to sit in?

More suggestions/ideas are welcome.

 

Mine is 21" deep. should I make it a bit closer?...say 8"?

 

I think that would work fine. I used to always get an idea on placement by shining it against a wall from close up and pulling back until the diffused spot seemed right for the tank and measuring how far back I was from the wall. FWIW

 

I think that would work fine. I used to always get an idea on placement by shining it against a wall from close up and pulling back until the diffused spot seemed right for the tank and measuring how far back I was from the wall. FWIW

 

the other day at a FLFS (new neighborhood) their halides were 3 feet up and their sps looked amazing. they were 250 watt 20k

 

oh yea man the bulb get's hot as crap like 400 degrees I think, the surrounding air is only like 100 degrees so it completely surprises you when you hit it.

 

Wow is the tank shallow?