How high to mount MH lights from water....

I am building yet another Light Fixture and was curious to know some of the different hights to mount Metal Halide lights from the water surface? I have 3 250W bulbs with reflectors to mount in my canopy. Any way to calculate this? Thanx.

 

6-12" is common for Metal Halides. As you get closer to the water, you have more trouble with salt water spray on your lights and increased amout of heat transfered to your tank. Also, you can have problems with high amounts of UV light depending on the kind of bulbs you are using and whether or not you have shielding between the bulb and the tank. As you go higher, the intensity of light transmitted to your tank decreases in a hurry. Part of the 'calculation' depends on how much light you need in the tank and how much PAR you have available. As an example, if you have 250W MH over a 12" deep tank with soft corals, ou can have your bulbs really high. If you have the same 250W MH over a 48" deep tank and want to keep SPS, you may well have problems no matter how close you get your lights. One additional factor has to do with the combination of your tanks width, the reflectors that you are using, and how high in the tank you are placing corals. Basically, you want to make sure that your bulb/reflector combination is giving you enough front to back spread on your lights to meet the demands of where your corals are positioned. Of course, the more your reflectors spread the light, the less light that is driven deep into the tank. Not a problem with 12" deep tanks, a bit of a challenge with my 48" deep tank. You are probably somewhere in between.

 

Yeah, my tank is 24" deep and I built my latest canopy to position the lights about 10" from the bare water. I also have 4 Actinic Blue 54W Giessman's T5's in parallel with the MH. I want to start up Hard Corals, any good advise on what type to start with? Thanks in advance.