It looks like possible fin rot by the dark ends of the tail. Can't tell if he has velvet or if he's biting his own tail. Shine a flashlight on him and if he looks like he is covered in gold, he has velvet (Google an image of velvet to see the whole gold thing) and tail biting is because he is bored so maybe try adding more plants or giving him a change of decor in his tank.
I'm not an expert so if one of the more experienced answer I'd say go with their expertise.
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