I agree with Salty..
Sometimes people get MH crazy and forget that some of us have tanks under 24" deep and under 75 gallons. hehe
I will be the first to say that an MH set up with tons of light is by far the most amazing to look at.
But if a tank is small all you are going to do is boil fish. lol.. ok maybe its not that serious, but the point would be that if your tank isnt that big, and you have good wattage in the form of PCs, VHOs, and NOs, then you should be fine.
Personally, if I ever decide to go with an MH, Im going to look into a bigger tank first. Its kind of like putting up Stadium Lights the size of a baseball stadium's over your back yard... thats just my opinion.
However, I have heard cases (could be wrong here) of carpets running around stinging the heck outta everything in their path. This may have only been a certain kind. Or I could have the name wrong but I thought it was carpets that can sting something nasty as they run around your tank finding a suitable place.