 Re: HPS balast with MH lamp????
Yes, he could use the ballast for his tank. The only thing you have to look out for is that most of the standard outdoor mh ballasts are probe start ballasts. Meaning, if you want to change the bulb to a higher kelvin rated, bright white or blue bulb you just need to ensure that is is NOT a pulse start bulb.
Watts per gallon is a very general rule of thumb? If the tank is deep you need more watts to penetrate the depths, if it is long you need more light fixtures or longer bulbs to spread out the light.
Par example' A deep tank like mine 25" I used 2 - 250 watt metal halides. On a shallow tank 18" or less 175w or 150w mh's would be sufficient. On a real shallow tank 12-13" compact flourescents are more than enough. Look at your options, listen to other people and make an informed decision that you can afford and live with. You don't have to go overboard with lighting like I did  I just wanted to make sure that if it was out there and I liked it I would have enough light to keep it happy.
Jim
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90 Gallon glass, 135#'s of rock and 2" sand. 2-250w 20k XM metal halides. 2-95 watt actinic VHO, 4 powerheads, Coralife skimmer. Maroon Gold Striped Clown, Blue Chromis, Kole Tang, Pygmy Angel, Serpent Star, Brittle Star,Gorgonians, Mushrooms, Yellow Polyps, Turbo Snails & Blue Legged Hermits. 20 gal sump with Caulerpa, Mag 7 return pump. | 
