Clowns stressed or something else?

Today I got a gbta (about 6" open) that I placed in the tank after acclimating it and I fed my tank as normal. I then ran some errands so I wouldn't be stressed about the bta settling in. I came home and the bta looks settled in and doing good. However I noticed one of my two maroons (both about 1 1/2 inch long) has a piece of his tail fin missing. The bta moved to a spot the clowns don't normally swim in so I'm wondering if the split tail is from the anemone or not? The only other thing I can think of is the arrow crab pinched him. Water parameters are salinity 1.024-5, pH 8.2, ammonia 0, nitrites 0, nitrates 0. Take has been fully cycled for 6 months and maroons have been in about 1.5 months. Tank is 36 gallon bowfront with two karolia 425's and a filter forget name that's for a 50 gallon aquarium. Do water changes every other week with ro/di from LFS.