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I would say maybe high nitrates as inverts are sensitive to this but check when you get back may need some water changes to get parapets back to what they should be
They leave their shells and die? Thats odd, maybe the pygmy is pulling them out. Hermits can die from parameter issues but for all of them to leave their shells is odd. They usually (IME) stay in their shells until they are too big or a snail is around for them to rob.