For future reference just because I haven’t seen this mentioned yet in this thread; long acclimations are good for fish bought locally but can be deadly for shipped fish.
If you get a fish shipped again, find out from the vendor what the salinity is that the fish ships at, make a bucket of fresh saltwater to match (or match your QT tank), and when you get the fish float the bag for 15 minutes to get the temperature to match then put the fish straight into the new container without any of its shipping water. Then you can slowly raise/lower the salinity to match your DT (if you’re skipping QT). This way the fish doesn’t get ammonia burns.
And, there are other diseases (flukes, velvet, perhaps more) that can attack the gills so keep an eye on your other fish especially since it sounds like the yellow watchmen wasn’t quarantined. You can make plans now for how to treat your other fish if they get sick and hopefully you won’t need to act on those plans but at least you’ll be prepared if you do have to act.