I have a 70w halide over my 2gal and I don't think it is too much light. That is 35watts/gal but I think there is a point when the watts per gal ratio is irrelevant. For example, if 70 watts mh keeps my frogspawn is healthy under 5 inches of water in a 120gal tank, why would it suddenly need less light just because I put it in a smaller tank?? The thing to remember though is that in a nano, depth has no real affect on light, because the amount of light that hits the BOTTOM of my 2gal is the same amount that would be penetrating only the TOP 8 inches of a 120gal (directly below the bulb). I am not a power crazy par-nazi or anything like that, the only reason I went with mh is because I believe that corals need a threshhold amount of radiation to be healthy. Also, halide is a very compact option for lighting - any other type of bulb/fixture would have ended up being bigger than my whole tank
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Finally, because of its penetrating power, I am able to raise my mh a lot higer above the tank, which helps with temperature control. 70watts of metal halide keeps my tank cooler than the 13watts of pc that it replaced
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- Chad