I honestly think 2 XR15s would be best for what you're trying to keep. It will fill the tank better because you can disburse the light more than you would with one XR30 that will blast the center. I'd start there... and if you want to add later you can. For a (somewhat) reference, I have 4 XR15s on a 48" long that's 21" deep and I get 250 par on the sand at about 70%.