alright, i already salted itDo you have a picture of the fish when you first got it?
How long did it take for the tail to look like that?
It looks like bites but could be the start of an infection (fin rot). If the water quality is good it is more likely to be bites.
You can add salt if you like. Use 1 heaped tablespoon of salt per 20 litres (5 gallons) of water. Keep salt in there for 2 weeks.
If it gets worse after a few days, post more pictures and increase the salt to 2 heaped tablespoons per 20 litres of water.