A 250w MH on that tank is way overkill, but can be used as long as you keep it raised up high enough. Seems like a lot of wasted electricity though.
I am not a fan of using old tanks, especially ones that have FW turtles in it right now, as a SW frag tank. Too many unknowns as to what went into that tank. It is a nice size tank for a frag tank, but a 20G long is also a very nice frag tank and only $25 new.