My male betta moon has this weird grey growth and then missing fin underneath and I'm not really sure what to do about it. I have not treated him for the growth specifically, because I'm not sure what it is, but I was treating him for fin rot recently because he was tail biting which lead to fin rot.
I noticed this growth a few weeks ago, though it could have been there longer.
Tank size: 10 gallon divided w/ 3 male guppies
Heater: 80 degrees
Filter: 10 gallon bio- cycled
Water changes: 40% weekly w/ added stress coat+
Betta age: about 2+ years
Here are some pictures... The growth is on the far side of his anal fin.. it is grey and there is missing fin underneath.