I would prob stop any and all preventative medications. the generally accepted practice is to only treat with medications when the illness has been clearly id'ed and you know itll treat it. IMO
you are only wasting money and supplies in doing preventative dosing while in qt. if you leave a fish in qt for 3-4 weeks, you will see any illness before it makes it to your DT, at that point you can id and treat. no sense in wasting the money and supplies blindly treating. also idk if the o2
levels would have cause this. I have a 10g qt setup in the same manner and any fish that have gone into it do just fine. also don't look to the other clown for answers, fish are just like humans. every single one reacts differently to illnesses. id be cautious trying to introduce a clown into a tank with an established clown already there. I had a snowflake clown in my 125 for several months when I introduced a clearly smaller gold-stripe maroon. (thinking the smaller size would offset the maroons bad habits) I spent 3-4 weeks nursing the maroon to keep him alive. eventually I had to just let them go. luckily for me I still have both and they are coexisting... I base that strictly on luck though.