I stopped using cuttlefish for my snails ages ago when I learnt that the salt content is too high in cuttlefish bone. I was advised to use eggshells (rinsed first) and my snails love eating those! So I'd recommend you throw away the cuttlefish and use eggshells for the calsium instead. I put in fresh eggshells every few days.
As for the mites... they are probably coming in on the soil in the tank? So I suggest you give it a good clean out ...I certainly wouldn't want mites on my snails.