i don't feed them garlic directly, but I heard of others doing it.
I soak all foods in garlic juice instead. There are three methods.
1) Fresh garlic cloves. Somehow mash these (I don't use this method, lol) so all the juice comes out...there is a kitchen tool for this, garlic crusher? Anyways place any amount of garlic cloves into a garlic crusher and allow the juice to come out into an unused medicine cup or anything similar.
2) Garlic juice from a store. Stores sell pre-diced or chopped up garlic, and often they float in garlic juice. If you want, use the garlic for some use so it doesn't go to waste. Take a bit into a medicine cup or etc and pre-soak.
3) (my way) I have McCormick's Garlic...it's a dried, power kind. However, the ingredient is ONLY garlic and absolutely NOTHING else, so it's safe for my bettas. I get the medicine cup and get some tank water. I fill it up 25%, doesn't take much. I pour in some of the powder...you'll just have to eyeball it. It should be watery, NOT too thick or pudgy (although the extra garlic may help if you want it to. That is just my own choice. Plus it seems the more physicaly powder garlic, the pellets will sink.) BTW they don't mix very well. You won't get a solution. You'll just see the water becoming more garlic-colored, and the powder somewhat there, but it's definitely enough to use just fine.