As mentioned, we need to know dimensions of your tank and a lot omore info on the lighting system...how many bulbs, what type, etc?
In general, MH lights have a throw pattern of 2 sq ft. Meaning, for every two feet of lenght in your tank you should have one MH bulb. A four foot tank would need two bulbs to be lit properly. Wattage of the MH lights is really going to depend on two things...what you wish to keep and how deep your tank is.