Even thought they make claims to be reef safe, in reality ther is no product that is reef safe. Trust me on this one. I put someting in my reef to kill cynobacteria and it caused my reef to crash killing everything in it's path just about. I do not believe there is any product that is reef safe.
As far as the garlic is concerned, I use garlic x-treme that I put into the food when I am feeding. I do this on a regular basis with all of my tanks weather they are saltwater or freshwater and my fish are very healthy. From what I have read and heard on this, the garlic makes some kind of repellant that the parasites do not like and cannot adhere to so they cannot stick to the host, that being the fish, and make them sick. I do not think that using garlic once the fish is already sick is going to be of much benifit. It would be in the best interest of the fish and your reef that you catch the sick fish and put him into quarentine where you can start a treatment for ich with hyposalinity or copper. You cannot use these methods safely in your reef because it will cause death for sure. If there are other fish in your reef, I would consider catching them too and putting them in quarentine too since if they are with the sick fish, they are probably sick too and need to be treated even if they are not showing any sympotoms at this time. I know. It is a pain to catch fish in a reef and it may require that you drain some of the tank and remove some of the live rock to accomplish this, but in the end it wil be worth it and you will not be putting your reef at risk.
Marinemom