I know that a Radion G3 Pro will reach about 30 wide and 36 deep, if mounted the (required for warranty) 8" above the water. You have to watch ANY led re moisture. They can die the death if a lot of moisture gets to them. Your 55 hex will have a lot of the characteristics of a wedge such as I have, and you might consider stacking your rock around an anchoring pole to get a 360 free space at the edges.