Garlic juice is from pureed garlic cloves. Puree the garlic, then strain the liquid out. Or look for a garlic press at a big box store. You can buy the juice in the grocery store or a cooking/kitchen store. The juice is a bit more flavorful than pure garlic.
I had a platy with the same problem....the little guy would NOT eat the medicated food. It's kinda messy, but I put some of the medicated food in a little amount (probably 1/2 teaspoon) of squished garlic (from the press) and rolled the pellets around. My theory was the scent of garlic would be on the food, juice and all. Not the most professional method, but the platy did eat a little bit of the "garlic rolled" pellets.