at this point to have to decide what you are willing to do and what style of reefing you want to do.
some have great success with out QTing and going for a natural biological style of reefing.
others QT everything wet that goes into the system.
both are valid approaches to keeping healthy fish., and like life, each has there pros and cons.
each has a methodology, tools and style .both has people that succeed and fail.
for the non Qting read
@Paul B book on reefing.
for the Qted approach get on the humblefish forums best place for QTing that I have found.
so here you are you HAVE some sort of issue with the tank. it might be just to fast to soon, OR it could be velvet or ick.
I would try to ID what going on so you can treat for that issue.
you have 2 options.
1.pull the fish put in to a stock tank or the like let the tank go fallow and treat the fish. you may loose some fish in the process.
2. leave the fish on the DT and treat there. you will need to know what going on so you can treat for that issue. you may loose fish here too.
(seems like you are leaning towards this)
comes down to what you are willing to do. what the other half "lets" you do.
is this going to be a FOWLR for ever? if so copper might be a good approach. ( with hanna checker ) but the rocks will adsorb the copper power so you never can do reef with the rock with out a ton of work to get it out.
things like UV properly setup help in management, as well as high quality fresh foods with live bacteria. I am not saying that is a cure more preventive just like in us humans where good diet gose a long way in heath but once there is a problem a green juice is not going to cure that cancer right away.
basically with QTing you are proactively treating the fish,inverts ect. and not doing it one is managing issues that might show up. but very common with the supply chain and how we get fish in our hobby.
there are vendors out there that Qt fish for you so you dont have to. yes its more $$ but cheaper than loosing a ton of fish or the work of trying to catch them.
I am leaning towards going that route for my 200/275 FOWLR but it is tempting to either not QT at all and go with adding real ocean LR/LS the more biological way, but man loosing a tankfull of fish is hard we have all been there.
anyway good luck!