Sorry to hear you are having so much trouble with your fish . The only thing I could possibly recommend is to get a cheap rubbermaid tub and put the fish in that while you try to disinfect the main tank, since you're right, you can't put them in the QT with your shark.
Assuming it is/was ich, in the future I'd definitely recommend the salt/temp. treatment. Some of the medicines they sell can be way too harsh for many fish and, IMO, cause more issues than they are worth except in extreme situations.
Assuming it is/was ich, in the future I'd definitely recommend the salt/temp. treatment. Some of the medicines they sell can be way too harsh for many fish and, IMO, cause more issues than they are worth except in extreme situations.