I have 7 pygmy corys in a quarantine tank. They've been in there since Wednesday. Just now I noticed that one of them has lost its tail fin; it practically has no more tail. The other fins look fine.
I cannot say for certain when this happened, because I haven't been looking at each of them very closely, but the cory's lack of a tail was so obvious when I looked at the tank just now that I find it hard to believe that I could have missed it over the past two days. I know at least that when I got them all from the fish store, they all had tails.
I have changed their water more than 50% twice since Wednesday, as the tank they're in isn't cycled and it's a bit small for that number (5.5 gal).
Anyway I don't have a spare tank on hand, so I put her in a plastic container with a low water level since she seems to be having trouble swimming. It's a very small container, though.
How do I go about treating this? Could it be anything else besides tail rot? Should I do anything to the larger tank with the rest of them?