Well if you have a jar of garlic with other stuff in there.. I dont know that i'd use that stuff..
If you get >FRESH< garlic just peel off all the skin and if you have a garlic press then you'll have a easier time... if not just dice the garlic as fine as possible and smash it to get as much of the juices out of it as possible and just mix in your food with it. It wont hurt a fish if he eats some garlic, but its not a great idea to give a fish too many large chunks of it.
If you want a much easier time... just get some 'garlic extreme' or similiar product that is meant for using with fish. Its all juice and saves you from wasting your mothers garlic