I think the garlic fad was more around getting some parasites to fall off due to the garlic seeping out of the fish or whatever the proper way to say that is. The parasite would attach, then taste the garlic, then drop. I've tried it years ago but don't think it did anything. That is when I started to feed shrimp and clams off and on to help with their immune system after reading some threads around the interwebs.
Now, if we are talking dogs and cats - in the past I had a few flea problems with my GSD and my goto was brewers yeast or garlic and it would rid her of the problem quickly. Dog, yard, and all. I think that was the idea behind Garlic and some parasites anyway. Hiking, dog parks, whatever - fleas can be a problem so that is in my bag of tricks. Fish - I have nothing I'm afraid but good luck!