You need to clarify the scope of your work. Garlic isn't "homeopathic". For some reason, garlic stimulates appetite in fish, and that can help weak fish build up their strength. While it is certainly a mild (very mild) antiseptic, and this might, conceivably, help a fish shake off an infection, again, this isn't homeopathy. There are chemicals in garlic you can identify as antimicrobial, and depending on the concentration, they may be beneficial. Much like tea-tree oil medications, in fact. Of questionable usefulness, but widely sold.
Homeopathy works by using chemicals that mimic symptoms, and then diluting them dramatically (indeed, scientifically, diluting them so far they're non-existent). While homeopathic medications can work, they work no better than placebos. Every single scientific test ever done with homeopathic medications has proven beyond any doubt that homeopathy only if the patient believes they will work. As such, it's hard (impossible) to see how homeopathic medications would help animals.
This isn't to say fish (or animals) mightn't get better after being treated with homeopathic medications, but I can guarantee that if you did scientifically meaningful tests, you'd find the numbers that got better would be just the same as those that got better without getting any medications at all. In other words, their immune systems and body repair systems would do the job. This is an overlooked issue in fish health, and there's some good reasons to believe that most aquarium fish are constantly fighting, without symptoms, low levels of diseases such as whitespot. It's only when exposed to poor conditions, poor diet, or some other set of stress factors that their immune systems are overwhelmed.
Cheers, Neale