FWIW... I was a skeptic before I was a believer. There is a HUGE caveat to garlic use though that renders most current preparations useless or nearly so: stability.
The garlic needs to be fresh... the principal irritants (to ectoparasites) from garlic degrade largely within hours.
I was scouring a collection of zoological journals on elasmobranchs searching for info on a species of shark I'm breeding (successfully
) when I came across some interesting papers on effective garlic use in sharks and rays. I followed the trail and references back as far as the 1950's(!) with papers describing the successful use of garlic in zoo/public aquarium fishes.
The really interesting thing, to me at least, is that back in the 1950's... 1960s... perhaps largely the 1970's - many folks cooked at home more often than not (read: used fresh ingredients). So when someone said "try garlic" back then... the aquarist would get cloves from the grocery store and press them without a second thought.
Nowadays... when someone says "try garlic"... we look for a pill or prepared bottle supplement (God knows how many weeks/months old). Hence (IMO) the reason for the now waning reputation of garlic use in aquarium fishes.
kindly, Anth-