If it is plain garlic extract I think you would be ok. However, the minced garlic we buy for people use at our house is packed in oil and I would not put that in my tank. You can crush your own garlic but in the end I think that would be more expensive than buying Garlic Guard or the like. The last bottle I bought was $12 and lasted over a year. At less than a dollar a month it's the cheapest thing I buy for my tanks.
