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If you have wall to wall front to back coral and are not able to mount them more than 16'' - 20'' above the water you will want 2 per 24x24. I had 4 of these on the 120g 4x2 tank and it covered very well. I moved down to a 90g and only have two lights and they are inadequate covering the 18'' front to back. Mounted 10'' above water.As mentioned by Eagle Steve, we have 3 over a 180 too. A good rule of thumb with these are 1 every 24x24, we like 90% ch1 and 20% on ch2