If it were me doing that tank with radions I would use 4 of the G15 units, equally spaced in a square configuration. In reality you probably get about 18"x18" of useful light out of them. The G30s will not give you complete from to back overage since, if they are like hydras you get roughly 24" of useful light in the long axis. Keep in mind the further out from the puck you get the more issues you will have with shading.
Coverage is the killer for LED, especially with the dense puck arrangement (radions, hydras, etc). I do have 4 non-HD hydra52 units on tank running at about 60% power.