What sort of snail would indeed be helpful. My mystery snails are obviously female, I have had some clusters hatch and others (from the first few clusters) not hatch, I am of the opinion they were not fertilised. The new clusters are obviously fertilised (despite my male being switched to another tank with one female and then passing on, the females can store sperm for up to six months). I now have something over 40 babies in one tank (decreasing as the fish pick them off and eat them, but I should end up with a manageable number).