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I've seen everything from <2" to 3'+It really does depend on several things.
The fixture power, height and width of the tank, optics, corals, how much spread you need. I'm sure there more things that could be considered.
I've seen anywhere for 8" to 18"
Generally, no optics are required below 6-8", beyond that, 80-90 degrees is good for 5-15", 60 degrees works in the 10-20" range and 30-40 degrees is used for anything above that. Most LEDs are 110-120 degrees without optics and need to be kept pretty low to avoid having a lot of light fall outside the tank, no more than 8" or so. My 90 is 24" tall, but the distance from the sand to the waterline is about 22". I initially set up my fixture without optics (110-120 degrees) and found that I was lighting the wall behind the tank and the floor on the other 3 sides, if I went more than 6" above the water. I have since added 90 degree optics and raised the fixture to 8" and I'm happy with the results. Different fixtures and different tank dimensions may have different optimal heights.My tank is 24 inches tall and my leds are 120 degree optics.
My tank is 24 inches tall and my leds are 120 degree optics.

