Perplexed about sailfin condition

[Simply Salty]

So my sailfin over the past 4 days has issues I don't understand. One morning I noticed a small area on his top fin. 3 days later it's this. He is very lethargic and at the bottom of the tank. Pulled him too the surface for the picture. I have never seen anyone pick or bully him until this morning and it was my scopas. They have been tank mates for 4 months.. introduced at some time. I'm searching for ideas. 2 year old Tank. All tests in range..

 

[Simply Salty]

Note: 40 min after this post, he landed in my anenome. The results weren't good. Would still be curious aon thoughts and opinions.