I never dose my tanks with garlic directly. I soak pellet and frozen food in garlic, selecon, and vitachem several times a week. More often if I am not being lazy. Seems like a more efficient way to deliver garlic to the fish IMO.
The food thawing in the garlic solution is already hydrated, it is not soaking up any garlic. How much of the garlic stays on the food when it hits the water?
Most if not all of it will wash off along with the benefits into the water.
Dosing the tank at feeding time will result in the same.
I thaw out the food, and rinse it well first then I drop in 3 or 5 drops of garlic extreme, a dropper full of selcon, 10 drops of Vitachem, and a tsp of Vitamin C. I let that sit for 15 minutes, then I add tank water to it, and proceed with feeding.