I would actually say fin melt rather than thin rot as I've just done a bit it researchMy Betta is in really bad shape right now. I was wondering if this is fin rot or a parasite which caused that. I suspect that he got that from a new betta I bought a few weeks ago which had the same thing but is now doing good (active and food dance).
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Just finished reading the rest of the comments glad I wasn't the only one to notice the difference, fin melt is apparently much harder to treat as not a lot is known about it from what I have readI would actually say fin melt rather than thin rot as I've just done a bit it research