It looks like a bit of a nasty thing going on there
I am not sure if it is cos she has been chewed or anything like that. Remember that bristlenose do not have scales anyway. It looks more like there is an infection in the tail.
The water looks a little cloudy, there are several reasons for this, the first and most benign is that you have recently sturred up the grave on the bottom. If this is the case then no dramas. It could also be caused by excess ammonia in the water, but you said that your levels are all ok so we can rule this out. Finally it could be a bacterial bloom in the water. This is commonly caused by excess food in the water, partucally vegetable matter (cucumber, zucchini etc. been left in for too long). 9/10 times this is harmless but if a fish has a wound then it can increased the chances of infection.
I would be doing a good clean and a good 30-40% water change to help the water clear up. I would also give the tank a dose of melafix to help it settle down.
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