That's because he's stressed out. Even though a tank is cycled, that doesn't mean you can skimp out on changes weekly. You should be doing at least 50% weekly, bi-weekly is okay depending. But he's suffering from some fin rot on his fins so to cure that you should do daily water changes of at least 25% to help him.
Notice his face looks white and pale, fish lose their colors when they're stressed out and unhappy which is what he is. He can gain his color back when he get's happy again. Water changes aren't just about getting rid of ammonia, nitrites and nitrates. There are other non-dissolved and dissolved organic compounds in the water that need to be removed and then water needs to be topped off so that he can continue to get the minerals he needs from fresh water.
I suggest just to start doing daily 25% water changes, small so it doesn't stress him out but starts to help him heal up. If you can, get some Stress Coat water conditioner to help replenish his slime coat and hopefully heal up those fins. The water changes alone with start to heal up his fin's since fin rot comes from bad water parameter's.