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Based on my tank which is 14" tall. I have my 150w MH DE HQI 6 inches from the top of the tank, 7.5" from the bulb to the top of the water. It is a total of 20" to the bottom of my tank from the bulb. I currently have a tubs birdnest about 10" from the bulb along with another piece of Hyacinth. However, I do have a tyree ponape birds nest at about 15" from the bulb and it is doing well also. I think at 15"'s though it does not have the color it should, but it is still quite small. I am also running under 20k. If I was you I will probably keep the bulb no further away then 6 inches from the water and this way you can keep your sps about 9 inches deep into the tank. However, to ensure good color and growth I would keep the sps no lower then mid tank and save the expensive pieces for the top 1/4 of the tank. Be sure they have really good flow on them in a chaotic movement and you should be fine.
I agree. My bulb (14k sunpaq) is about 6" from water surface. I have a mixed tank so positioning can get a bit crazy, but in general, everything thrives nicely.
people always told me 6 inches from the water...that way the light can spread in the tank instead of being concentrated in a specific area they are all correct