Sponges don't typically do well in reef tanks... Occasionally they may survive, but not often. Also, IMO the nicer they are, the less likely they are to survive.
In my first fish tank, my fish were acting weird and had what looked like little white spots (ich). I fed them garlic at first and they survived. I stopped feeding them garlic and they still survived. My second tank had the same problem, but I didn't feed them garlic and they survived.
I'm also quite skeptical that hobbyists are actually capable of diagnosis ich in the early stages...
Now I think the white spots might have been sand or bristleworm spikes and the fish just weird.
I think it's fairly well established that Allicin has SOME immune-stimulating effects in mice and rats.... for whatever that's worth. Any link to fish and ich however, is as greenbean36191 suggested, purely anecdotal. Doesn't mean it's not true, just means its a guess.