You want to use only kosher or aquarium salt. Regular salt is iodized, it contains iodine that can be toxic to fish. Aquarium salt and kosher salt are free of iodine and potassium that also gets added in to salt sometimes.
I'm not totally sold on the garlic thing myself. It has a chemical in it that is mildly antiparasitic, but it is very weak. You would need to use something like 90x the amount of garlic as you would use malachite green for instance, if you were treating a parasite like ich. Simply because the garlic is "natural" doesn't mean that the antiparasitic doesn't have similar negative aspects to malachite green--it is just a much, much lower concentration. Bettas never really encounter either electrolytes or garlic naturally--so I would only use salt/garlic when there is a medical need. If you are interested in the positives/negatives of using salt and garlic, I suggest you research them.