I crush fresh garlic in my homemade fishfoods.Also blanch veggies with crushed garlic before feeding.huge difference in how much the fish eat with the garlic.
Watch out for additives in garlic powder, sometimes they include an anti caking agent. Just get a clove from a bulb and squash it with the back of a spoon, remove the skin and hey presto there you go. If you are feeding your fish meaty foods you can just rub the garlic on it to enhance the smell. If feeding loricariids you can push the crushed garlic into the flesh of the fruit or veg.