Andrew, where are you getting that garlic causes skin disease from that article? If you do a search for "skin" it's not even used there.
Also, the conclusion of that article is not "it does more harm than good". It is "use at your own risk, the controlled studies are still out, and until then, there is only anecdotal evidence, not yet scientifically proven against marine ich". There is evidence allicin has other effects on fish immunity, but the article is just arguing against the myth that its use against marine fish diseases has definitive proof yet.