[pogodzib]

Since 2x65w bulbs over a 20" tall tank isn't very much, you'll probably want to be in the 10-12 hour/day range, depending on your corals. Certain corals may not need the lights to be on that long, but others won't be able to thrive no matter how long those lights are on. If you are sticking with the lower light demanding corals, such as shrooms, dull colored zoos, polyps, and softies, then 10 hours per day should be plenty fine. If you have some LPS, you may want to go a little longer and keep them higher up in the tank.