I once had an overabundance of snails, so I cut back the fish food, and the
snails began to die. Well now in my 2 - 3 inch deep gravel, there are
hundred of snail shell remains. Will this hurt my fish or plants?

No. If your water is soft, the snail shells will eventually dissolve. They're
a good source of calcium.

Worse is what tons of dead, decomposing snails could do to your water
chemistry. Though if they really starved to death, they probably ate each
other. Anyway, if the shells are all empty now, you have nothing to worry
about.

Leigh

http://www.fortunecity.com/lavender/halloween/881/