Garlic seems to have somewhat of a preventative effect when it comes to C.irritans, and may be of some benefit in helping the fish recover as well. That being said it is NOT a reliable stand alone treatment method, and is certainly not a replacement for hyposalinity or medication. Garlic seems to do the most good with hardy species that have a decent shot of fighting off the parasite in their own anyway. With fragile species, especially when the immune system is severely depressed, I’d venture to state that it almost never works.
Marina has given good advice, but I wouldn’t rush to assume that hypo is unlikely to work. There’s no reason to assume that you’re looking at a strain resistant to lower salinities for one thing, for another this method has a proven track record.
BTW, hypo is administered at 1.009, lowered incrementally over a few days.