Help my fishy has a gill thing

It's a gill infection. Could be caused by ammonia or PH burn. Have you checked your parameters lately ? Might want to do a water change and if you have a guarenteen tank you could put the danio in there to treat him. Extra aquarium salt and an antibiotic treatment should help. Or, you could just do a good water change and keep it extra clean until he heals up. Sometimes ultra clean water and extra salt does wonders to heal up a fish and isn't as stressful as some med treatments can be. Putting him in a smaller tank for awhile could make doing this a bit easier.