How bright do you need it to be? Are you wanting a 4' fixture? Or 2 36"'? Or 3 24"? 
Is the tank single or double braced?
Tank is double braced... as I mentioned above... I am open to what someone might have at this time.
I like my tanks BRIGHT.
I have a 4' single bulb florescent in it now. I want to at least add, if not replace it.
For my 4'... I have 4 T5 4' bulbs on it and like that look.