I've noticed several shells with holes in them. All are aged, meaning they have been in my tank for several months to several years. For now I've only seen this on snail shells (empty ones) and this oyster shell:
Google lead me to acidic conditions and or serious lack of calcium. Bot have been tested and are fine. What else could be causing this?
I do have a few mini starfish. Most never move, I see their legs poking out of rocks. I spotted a milky white looking one tonight. They eat holes through shells?
No limpets that I've ever seen in my tank.
I do have 2 peppermint shrimp, I've seen their skeletons a couple times, so yea they have molted. I've never seen hermit crabs molt, but I suppose they do also. I have between 15 and 20 hermits that range from 1/4" to 3/4 inch.