I have been monitoring my bettas tail the last week because I noticed since I got back from a weekend trip his tail looks different. When I got back from work today it looked a lot worse! I don’t know what it is, Is this fun rot or something else? He is in a 20gal tank with a filter and heater, love plants, a nitrate snail and a Cory catfish (they have been together for 3 months now). I change 40% of the water every 20-30 days and Ph and Nitrate levels are normal. I don’t know what to do! He is acting fine and swimming normally but looks like something is definitely wrong with his fins!