Garlic is a classic "natural remedy" that is used by more experienced aquarists, especially by fans of Discus in their "pimps" for its antifungal, antiviral, vermifuge, antiparasitic, antibacterial and anti-infection.
And 'expressly used for example against intestinal bacteria such as' Escherichia coli-and generally not as curative but in the Prevention of infestation of internal parasites.
Is sometimes mistakenly associated with the fight against the "disease of the hole", typical of large cichlids, not only discus, but I do know garlic NO has no effect against the disease of the hole.

Some commercially available for some years now many products containing garlic for aquarium use, whether that special liquid food, so instead of "impuzzolentirsi" too hands and sitting peeling and squeezing a clove of garlic, I recommend buying the food that contains ALSO extract of garlic, so as to guarantee a minimum contribution to the diet as "intestinal disinfectant," as a stimulator of appetite and the entire digestive process ... and finally as a vitamin C supplement.

HOW MUCH?
If you do NOT opt for foods or other additives "fattiapposta" and rely on the do-it-yourself, then we can try to crush a clove of garlic, after deletion of the green soul, and use the (few) drops produced by wet granular or food that is thawed. The granules absorb even better in my opinion ... though we mash in a nice home for our fish, the better .... but I do not know how many non-discusofili have this habit.

WHEN?
I do not know ... is a form of prevention and also depends on what causes odor in your aquarium and your level of endurance.
As a guide, I would say once every 2 weeks may be an appropriate timing.