BINGO! Post #20 and we finally have a picture developing. Firstly, that fish must be as miserable as can be in that size tank, it can't be helping it's situation at all, have you any plans for an upgrade should it pull through? You say nitrate was "an issue before". What level of nitrate (before) are we talking about, and how long was it swimming in this nitrate infested soup?
We also have no background info on the previous set up it was in prior to you adopting it. For all we know it could have been exposed to heavy nitrate in the past too. Also, the nipped fins point to other potential issues, but for now i'll stick my neck out and say it could be a long term water quality issue which is finally catching up with it.
I hope i'm wrong and that it's something and nothing that can be treated. But whatever, it needs a bigger home! Good luck.
