as much as you can use without overheating the tank. a three foot tank is a little tricky. mh covers a 2 foot area. so one might not give you complete coverage but two might overlap. this could be good for a high light area. If you are concered with upgrading in the future i would go with the bigger lights. that wat if it gets too hot you could run one 250, but if you upgrade to a larger tank you could easily just add another 250 or two.