Loss of a fin??

Question
I have a freshwater aquarium with a few different types of tropical community fish. One of my fish (an orange and black one, I'm not sure of the name) appears to have lost one of its fins, the right one specifically. There is a sort of white area where the fin used to be. However, there appears to be a little nub or a very small fin in its place. My question is can the fish regrow its fin? Or is this just the remnants of a lost fin still attached. There aren't any aggressive fish in the tank, but there is one other fish like this one and it appears to be pregnant. Perhaps they mated and the other one became aggressive and tore off her partner's fin? I don't know, I was doing some light tank cleaning a few days ago and adding more water, moving gravel around but I don't think i sideswiped any fish. Anyway, I'd like your input on this if possible. Thanks a lot!
Mohsen

Answer
Hi Mohsen,
If the edge of the fin has turned white this is fin rot.
Fin rot is caused by poor water quality. Test your water for ammonia, nitrite and nitrate. I would do daily water changes because the fish will re-grow its fin when water conditions improve.

Regards,
Chloe.