Jack Wattley did extensive work using garlic to rid Discus of intestinal parasites and worms. I'm paraphrasing a bit from memory, but here goes....
He was able to expel intestinal parasites from Discus using only "fresh pressed" and finely minced fresh raw garlic. He stressed this point as he was not able to get the same results from any sort of garlic powders, extracts or oils.
Unless something has been done in the ensuing years, that I have not seen, I would say that commercial garlic treatments are not proven for anything beyond stimulating a feeding response in some fish.
Edit- Found a piece of the longer article that was in TFH...
Discus Page Holland has lots of articles about the Discus fish. With now more then 120 breeder surveys online and a free discus screensaver. Also more then 1600 pictures available. Mpeg movies. Several great shows online
www.dph.nl