Peppermint shrimp will eventially kill a bta if left in the same tank. I had this problem. I lost a large bta, a blue carpet, and a small bta, thinking it was other things, water quality or something else. I checked and rechecked my parameters in the tank but everything checked out o.k. As an experiment I purchased a small sebae anemone and put it in the tank. As soon as I put it in the tank the peppermints were around it picking but not really tearing at it. I left it in the tank and got up in the middle of the night to find it shrank up and the peppermints tearing at it. I pulled the bta out and put it in a 20 gal. Its been about a week the sebae is starting to look like he will make it. As for the peppermints I had 8, I took them to my lps and traded them for a coral. By the way I had a pair of large Maroon clowns in the large bta. They would not let anything close to it during the day. But I feel the bta was being picked on at night the same as I had found the sebae.