Don't buy any more of them.
Mollusks cannot survive in home aquariums. There's not enough food for them & basically you're just letting them starve. Once they finally croak after a month or two.... they're going to foul up the water & poison everything in your tank.
Also they NEED sand to burrow into, the gravel in your tank aren't going to do anything other than leave them open for attacks from other inhabitants which will cause them to "CLAM UP".
In the wild, mollusks are buried right underneath the surface so they can feed off the microbes/algae in whatever body of water they live in.
Extremely useful invertebrates in the wild, but in an aquarium because of their lack of mobility to locate food for themselves they're completely useless & a waste of money.
Invest your money on LIVE aquatic plants instead of these slow moving shelled bio-hazards.
Last time I checked those things were were $2-$3 each.