Ick is curable if you get started on it. However, you're not saying if you can move your sick fish to a hospital or not, which is why I suggested a FW dip [since you can't do hyposalinity in your current tank].
Also, if you are dealing with ick, giving any fish to anyone else or returning them to the LFS wo telling them about the diesease, could and most likely would result in ICK problems in whatever aquaria your fish are placed.
Yes, RO water for a dip, or distilled are fine. You must be sure that the PH and temp of the FW is the same as your tank. Better, though, to setup a hospital and go with hyposalinity. Garlic: you can use freshly chopped garlic, making sure that when you chop up, you retain the juices for your fish food, or you can buy pure garlic extract from your local grocery or health food store. Be sure that if you buy bottled garlic that all you are getting is GARLIC, and not other additives---read the label!
Good luck!