MH lights can't be judged by watts per gallon, in my opinion. That is because those lights are almost always suspended some distance above the water, and because they either have a lot of spill over light or they don't cover the entire tank. (The old round peg in a square hole problem). Then, there is the problem of limited sizes available in MH bulbs - 70 watt minimum, but 150 watts as the next bigger size, etc. Someone with the interest could devise a poll with the right questions and find out from the wide range of experience with MH lights, just what a "high", "medium", and "low" light tanks means with MH lights.