I think garlic as a form of medication is a bad idea. To combat ich, it's best to use either hyposalinity or some form of copper treatment (not both). From what I've read, garlic might help prevent a fish getting ich, but does nothing for fish that already have it. It's entirely possible (and likely in my opinion) that garlic doesn't really do anything to prevent/cure ich.
Your UV sterilizer should help with ich, but won't entirely prevent it. Ich will almost definitely spread to your other fish. Do you have a quarantine tank? Does your fish tank have invertebrates in it, or just fish? If it's just fish, you can treat that tank with either hypo or copper. Both treatments will kill invertebrates, though.
