Sand on the rock is no problem. Just waft/blow it off (a turkey baster is good for this).
Suggest you get an STC-1000 (temp controller) to keep the temp stable, and wire it up for two heaters. You need two that can each keep the tank warm (in case one breaks). You need to increase temp slowly, as a jump of 3 degrees will stress the fish. Marine's need stability over anything else.