My maroon clown got mouth fungus.. she was biting her anemons tenicles and at first I thought she was stung. I observred her all day and she started getting worse. It turned into fin and tail rot/bacteria infection.
Her mouth had white funk, her cheeks swelled, holes appeared in the middle of her fins, she swam at the top for extra oxegen, and wouldet eat. Im telling you this so you can look for signs.
Either way, and im a newb, but Your probrobley have to quarentine him and give him a medicine. Im sure someone will chime in with more knowledge
My maroon is in qt now and just finished her treatment of maracyn - 2. shes perfectly healed