Frayed Fins - bitten or ill

One of my Angels has frayed fins. She was fine until about a week ago. I have only had her for about three weeks. She is bullied so could be bitten. My question is:- how can you tell the difference between bitten fins and fin rot or could it be something else I have not thought of? Any help you could give would be much appreciated.

Could you test the water and post the results? This and a pic of the fish may help. How often do you perform water changes, and how much? It may be a water quality issue, but if she's being bullied that's a more likely cause.

Bullying leads to stress and stress can lead to low immunity so it could be a mix of the 2, as Helen says @water test results would help to get the the root of the problem
Lisa

thanks for helping.

I forgot to say that my tank is 165L and the Angels are currently about 4 to 5cm excluding tail fin. There are only three of them. Eventually I would like to get two more. Is five okay in this size tank?

water test results are:-

PH 7.5
Ammonia 0
Nitrite 0.1 if that
Nitrate between 5 and 10

I will try and post a pic (don't know how to at the mo).