Sorry to hear as well. I'm fairly confident that the small majority that QT 100% of the time are those that have gone through something like this. I recently had a large outbreak of ich, and QT'd all my fish. I only lost a kole tang, but every single one of the fish came out looking better than when they went in. Although it's tough, I would recomend catching all the fish, treating with copper, keeping the tank fallow for 8 weeks, and then QT everything that goes into your tank from here on out, from fish, to frags, to anything that goes into your tank. Although it was a burden for the first couple of weeks, I soon found the process rewarding.
I'll keep my fingers crossed for your yellow belly.
