I have a 100 gallon reef tank, and one is plenty. Depends on what you have in there. SPS dominant, do it. LPS, maybe not. Shape of the tank would mean something too. Mine is a 30" x 30" x 24" LPS/anemone tank. I couldn't see having two on opposite sides of the tank in a cube like mine. Might not be as bad if your tank is 48" or more L to R.