So what's the longest tank that having only one Radion XR30W Pro will work just perfectly? For example, would one work on a tank the size of the Red Sea Reefer 250?
Several factors come into play. Height of light over the waters surface, livestock kept, dimensions of the tank. Say you wanted to have a mixed reef in the Red Sea Reefer 250, I'd recommend 2 simply to prevent shadowing and even par distribution across the tank. It's usually recommended to place 1 Radion every 18-24". Being the RSR 250 is 35", I'd recommend 2. You could get away with 1 if you didn't mind some shadowing and only kept fish and soft corals.