What the heck...

[MINIfish]

Is this??? It's on both pectoral fins and nowhere else. I have tried maracyn, maracyn 2, mela/pima, salt, maroxy, and nothing seems to get rid of it. The water tests fine and none of the other fish have it, just him. Any ideas on what it could be?

 

[guppy]

There are two possibilities that I can think of. The first is encysted nematodes, you can try praziquantel or de-los for these. The worms are diagenic and do not spread to other fish.
The second is dermcystidium, a disease I have only read about. That is a fungal disease and has no known treaTment. It ids not generally dangerous but can spread to other fish if if nodules rupture.