IME it helped when I had a light infection but when a severe infection occured some time later it was completely inaffective. Here's some ood info I found:
"Using herbal remedies to combat Cryptocaryon irritans or ich has become more commonplace over the past few years with marine aquarists. Garlic, in particular, has become popular. The foods are soaked daily with garlic prior to feeding the fish for several weeks to combat ich. Unfortunately, evidence to support garlic's effectiveness or lack thereof, for the treatment of Cryptocaryon irritans is anecdotal (Bartelme, 2003a. Cortes-Jorge, 2000). It seems to work in some cases, especially with light infections, and not in others. However there is some evidence to support garlic as a treatment for some other types of parasites.
An extract of garlic, allium satiyum, has been reported to eradicate trichodinids (Madsen et al. 2000). It has also been demonstrated that garlic extract kills Ichthyophthirius multifiliis, at the theront stage within 15 hours. Ichthyophthirius multifiliis is the freshwater parasite that causes an infection that is often referred to as ick.
The extract was prepared by crushing fresh garlic cloves. The juice was then added to the aquarium at a dosage of 62.5 mg/L to kill theronts.If more than 50% of the theronts died the concentration was recorded as effective ((Buchmann, et al., 2002). A dosage of 570 mg/L was required to kill 100% of the tomocyst stage of Ichthyophthirius multifiliis. Preliminary tests using allium satiyum indicated that it has no effect on trophonts in the skin of fish (Buchmann et al., 2002). The amount of active compounds varies widely among the different varieties of garlic and the use of garlic, by adding it to the water, may prove to be unrealistic ((Buchmann, et al., 2002).
The mechanism of host location for Cryptocaryon irritans is unknown (Colorni & Burgess, 1997). It has been suggested that garlic, when administered orally to fish, may interfere with the parasites ability to parasitize by camouflaging the chemical attraction of fish (Cortes-Jorge, 2000). Chemical attraction requires further investigation (Colorni & Burgess, 1997)."
http://advancedaquarist.com/index/
my blue tang got ich when he was first introduced, he ate my hommeade food and got better by him self. But i heard that is possible too with light cases, so the food might have not been the cure.?? SO basicall i dont know if it works or not, but i feel better having some garlic in my food. 
