if i understand this correctly, the extract has to be assimilated by the fish for it to work. and, like you said, it acts more in camouflaging the fish rather than killing the attached parasite, right? so, regardless of which protozoan you are dealing with, it should be more of a preventative treatment rather than a cure?
i'm just really intrigued by this mode of treatment. in hindsight, when i was researching about tropheus, i've come across alot of discussions about garlic extracts preventing bloat and other diseases of this fish. to think that that was about a year and a half ago really seems to give credence to the claims of its proponents.
If you find they are spitting it out, then you may be using more than needed.
it figures... i guess i'm putting quite alot of garlic extract in their food after all.