Yep sand is not a problem for snails
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What is far more likely is that they're starving to death- this is the way a lot of aquarium snails go (particularly apple snails). They don't compete with fish for food well and if there isn't another source of food in the tank (edible algae suitable for the snails, dying aquatic plants, uneaten food etc) then they'll just gradually die off.
Personally i found the best solution was weighted cucumber- it takes a while to go off in aquarium water and so doesn't quickly foul water quality if uneaten, but its also stable enough to last long enough for the snails to get a good munch on it. My apple snails have thrived on cucumber (i also feed them algae wafers as they only share the aquarium with swortdails, amano shrimp & a para pleco) and the tanks substrate is fine natural sand too
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