Have had freshwater fish for over 20 years newer to saltwater about 2 years or so. Have a question about my mandarin. He’s got a weird growth on his fin and it seems to limit his swimming.
It's a bit hared to tell, since the picts are so small, but my first guess would be lymphocystis. Here is an article on it (offsite) - https://edis.ifas.ufl.edu/fa181
The other possible thing that comes to mind is a fish louse.
In either case, since this is on the fin, you might be able to pluck off the growth, if you can easily catch the fish.
