do not form) by a layer of black sand/mud, sure it is an obvious sign of what is going on. as a bubble though on it is rush to the surface, i do not think it is going to do any damage to anyone other than our nose unless we keep it in the water column till the whole thing dissolves. like your findings, it is not a concern. as for having a sloped substrate, ... i would guess your snails will try to level it o