+1 Nerite eggs. I have a handful in my tank. They litter with eggs periodically. If they hatch their progeny probably become part of the foodstuff for the rest of the critters, but in any case the eggs dissapear after a while. And when they're not laying eggs (which fascinates my wife - always a good thing to keep her involved!) they are chewing algae from difficult to reach spots on the glass and rocks with little of the collateral damage I had with crabs and other snails.