As you can see in the various replies, it can be anything in your tank. My best guess are the inverts. Are you feeding them? If they are starving badly, they will turn on your softies for dinner. Maybe try feeding them small bits of raw table shrimp or scallop. I usually stick a small piece on the end of a pointed stick and offer it to them. They'll grab it and pull it off the stick and I repeat for the next invert. This way, I can make sure everyone eats and also keeps the tank clean.
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George
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