You can use fresh garlic cloves that you can mince, making sure to capture the juice that is extracted. You can also use garlic juice that you buy at the grocery store, as long as it only contains GARLIC. Look at the ingredient label. It must not have any other ingredient.
There are also garlic mixtures that are sold in the hobby, such as Garlic Extreme, etc. This is more expensive.
If you mix up fresh seafoods then store in the freezer, then you can also mix up the garlic in the food when you food process/prep the food. Otherwise, you will need to allow the garlic soak to in the food for at least an hr in the refridgerator. Flake foods does not work very well, so your options are mostly fresh foods, or frozen hobby foods.