You could keep about anything you wanted with a 175w. The watts per gallon rule is kind of a joke if you ask me.... there are many things to consider such as type of lighting, height over tank, depth of tank etc... so you cannot just say 5 watts of anykind of lighting per gallon will do the trick. If you get something that is a little more light demanding I would just place it a bit highet up in your tank. BTW
, is your 30g a 3 foot tank? If it is one single MH
maybe cast a bit of shadow on the extreme ends of your tank?