I read every article about ick I could get my hands on. Only one article (ATJ's Marine Aquarium Site "Marine Ich" suggested there is a possibilty fish can acquire an immunity to ich. The aquired immunity developed only after more than one exposure. If left in your display tank..yeah a fish or two might survive the first exposure, but in my humble opinion chances of their surviving a 2nd, 3rd, 4th exposure is pretty low.
Damage to the fish is already done when you can see on the fish the white, saltlike ich...these are well fed adults (mature) ready to leave their host (your fish) for their reproductive stage. If you do nothing, the fish that survive this outbreak will be subjected to a 2nd exposure...a 3rd exposure...etc.
Don't be a dummy like me. Get your fish into a qt tank...leave your display tank empty of fish for 4-6 wks. I don't know how much damge your pocketbook can stand, but after the loss of $200 (not including shipping) worth of fish & the little blenny being hit with ich a 2nd time, I finally got wize. The blenny is in a qt tank and my display is now fallow.
Denial is a terrible thing...loss of fish wise and pocketbook wise.
I'm just a lowly reef-keeper so please do not be offended when I say don't be stupid....take the advice offered to you by people with more knowledge of and experience with ich than me or you....I have experienced what you are now experiencing..I have walked in your shoes. If you are not willing to have a qt, then do your fish a favor & give your fish to someone who will put the fish into a qt and let your display tank be fishless for 4-6wks.
I agree that the qt is a remedy for ich, but it does have it's
limitation and I totally disagree with leaving a fish in qt after the mature forms of ich have left the fish. I think throwing the baby (babies to be) out with the bath (salt) water is very important at this point. I do think a 2nd qt is necessary to prevent reinfection. I do not think copper, hyposalinity, garlic, etc are useful ich treatments...too many things can go wrong...I wouldn't waste my time or my money. just my 2 cents....dr