Cory and back fin spot

[izzy]

This I can't find on the net for some reason.

One and only one cory has this for many months now. No other fish has this (neons, lemon tetras, plecko etc...)

It is a while spot on his back fin close to where it attaches to the body (towards the back). It is still on a fin as when he moves the fin left and right the spot moves. It is about 1/8th of an inch (I have glass aquarium so it is probably smaller then that) in size. It is not really a spot, but more like a cyst of some sort.

Cory behaves just like others do and the tank is set for over a year and I use quaranteen tank whenever I add new fish (haven't added any on over a month and he had this before that).

 

[QTOFFER]

It might just be what you said: a fluid-filled cyst. I doubt that it is a parasite like anchorworm or ich since it has affected this one fish for so long. Unfortunately, not much you can really do about it.
Could also be a tumor. If so, hopefully it is slow growing and does not impede the cory's movement. It sounds like it's not bothering him at all, so I'd just keep an eye on him.