hey man sorry to hear about this, it may be one of those times where you need to ask yourself if you can help this little guy or if its best to pass it to someone who has the right set up and can help him for you,its def a hard decision.
even with hypo salinity if you add him back to your tank in a couple months theres still a huge chance he can get it again.
you need to ask yourself if a $100 fish is worth jeopardizing your system with disease.
as it stands now its the only fish showing the signs right?? well if thats the case i believe if you keep him in your display he will add stress to the other fish.
remember copper will help for sure but its hard on weak fish and you will still have to solve the underining issue for the ich in your tank.
a freshwater dip can help for a very temporary relief but its also stressfull....catch 22 i guess...
if you must keep it put it hyposalinity and soak everything in garlic extract ,he needs to eat and eat lots...keep the lights out or low and try not to stress him out.....lots of hiding places and keep your water clean.
good luck mate i hope it works out for you...
my personal advice is to pass it on to someone and cut your losses, dont add any fish for a while and observe your other fish
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