I have a flat block retaining wall behind our patio pond which we want to put a water falls on to connect it with the pond that is at the top of the wall. We have thought of several possible ways to do this without trying to stack rock straight up as we can not attach it directly to the wall. We are thinking of use the blue or pink insulation foam and carving the rock formation into it. This will keep the cost and weight of the falls both down. My wife and I are both somewhat artistic, and think it would be fun and interesting to do. Now the question is what kind of paint or stain can we use as the ponds do have gold fish and large koi in them. Is the regular spray paint you can get safe, or is there something special we should use.