Change the bulbs out and what are the dimensions for the tank. I run 2 bulbs currently over my 40B, but am working on adding two more. On a 50, I would go with 4 bulbs. This is due to my sps I have in the tank.
If you keep the brain type LPS or mushrooms, you can get away with less light. For example, with my current 40B, I have sps, but high in the tank. My brains (lobo, scoly, platys, favias) are all on the bottom and doing great.
My 20g on the other hand has 2 bulbs (T5 also). It has my neon leather and some shrooms. They came from my main system that is MH and T5s, so are used to more light.