Hi and welcome to the forum
The tail is definitely damaged and appears to be bitten by something (as suggested by Deanasue).
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How long has the tank been set up for?
How long have you had the fish for?
How often do you do water changes and how much do you change?
Do you gravel clean the tank when you do a water change?
Do you dechlorinate the new water before adding it to the tank?
What is the ammonia, nitrite, nitrate, pH and GH of the tank water?
What sort of filter do you have?
How often and how do you clean the filter?
Have you added anything new (fish, plants, wood, etc) to the tank in the 2 weeks before this started?
Are there any other fishes in the tank?