If you have hermits than I would suspect it is them. If you do not give the hermits empty shells to grow into they will steal your snails shells.
And I found this out the hard way.
Now, my hermits have plenty of shells, but if not, they will pick apart the snails in no time at all. My hermits have not touched the astreas though, and went straight for the turbo snails. Perhaps their shells were prettier.
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