Indeed, a single 65w bulb would be plenty for a 20. You're actually running about twice the usual wattage for high-light plants. That type of light is designed for SW tanks that also need actinic light. You could simply take out one bulb.
Both 65W bulbs are 6500K freshwater bulbs, but yes ONE is plenty in this tank. I got it for a dirt-cheap price from a friend, so I put it (or rather half of it!) to use!
