I'm actually looking into this right now for one of my fish... apparently, garlic extract is an appetite stimulant, and supposed to be a general health booster. Its totally homeopathic, but I happen to believe that there are some good things to be found in homeopathic medicine.
It can be used by soaking food in it, or if the fish is just not eating, can be dripped in the tank.
There are a number of brands of the stuff out there, my LFS has Kent "Garlic Extreme"... thats what I'll be picking up tomorrow to try... seems that some brands also have a lot of vitamins in it... I would be interested in giving that a try if I had fast access to it.
From another forum, someone suggested just crushing garlic yourself and straining the extract through muslin... but i dont have the measurements of how much is safe to use at a time. Also, they mentioned to *not* use the jarred garlic as it has preservatives and other chemicals... I would think that would extend to powdered garlic too.
Hope this helps!