I have one of these. Running at 50% channel B and 30% channel A generally. It's really bright over a 29g high. New tank so I'm seeing algae directly under the light. I'm needing to add lights on the end is my only issue it's on a stand 7" above the water and the last 4 inches on the sides are somewhat dim. Just adding 30watt 5400k photography CFLs on the ends see how that goes. So on a 75 two should be fine, especially hung. But for better spread I'd go with maybe the 30" or three lights if you find it's not the best spread on your 75.
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