As
@nereefpat said - tangs can do this - with stress, lighting, time of day, etc (change coloration) - and to me its not just the 'tail' thats darker - its the entire coloration of the fish itself.
Having said that - to answer you question - I do not believe its 'tail died'. To me it looks like a fish thats in too small of a QT tank perhaps? But lets pretend its tail is paralyzed or died, etc - to me it looks like a systemic process thats not going to easily reverse - unless you can come up with a cause.
1. What are your parameters - ammonia, nitrate, etc.
2. What other fish do you have - is the water well oxygenated?
3. I assume you're using some kind of medication? What, if any? Are you sure the dosing of anything you are using is 'correct'?
4. How long have you had the fish? How big is the tank?
5. Any other symptoms - like fast breathing, etc? Going to the top of the tank, etc?
6. Is he eating?
Discus - of course - can do this type of color change when they are 'dying'. So - I'm not trying to minimize your issue -- and to me - its the swimming thing thats the most bothersome to me (i.e. if you touch it it doesn't move). I hope this helps - one way you will know - is if the color returns to 'normal'. during the day.
