Well you could do up to the 36" fixture, but man that's pretty expensive at around 230 + shipping. You'll have around 60 watts, and I'd guess probably in the ball park of 3500 lumen-which is nice and bright-probably over kill for such a tank. You might want to consider the finnex Ray II, should be a lot less expensive, and will yield high light on a 12 Long. The beam angle depends on how high over the tank you want it. If you can tell us that, I'm sure someone can recommend an angle.