Well without seeing water parameters ie. ammonia, nitrite and nitrate, I can only guess at what's wrong.
When my males bite their own tails, the ends sometimes look bloodied, which is what it looks like has happened with your boy. The pectoral fin damage could also be bites from the danios.
Just because when you're around they don't appear to be harassing him, doesn't mean it isn't happening. My males have torn their tails to absolute shreds in the span of a few hours.
The only other explanation I have, is that your water quality isn't the greatest and you have a build-up of something like ammonia or nitrite that is affecting his fins.
However, considering he looks fairly healthy and you would be seeing other symptoms like flashing, clamped fins etc. in your other fish, I would be more inclined to think the danios were to blame.