I find this interesting. I have read many articles that stated treating the water with metro is useless, as it is fractionally absorbed from the water column.
I also had the opportunity to ask Paul Loiselle about this at a lecture of his I attended and he too agreed that metro is best administered orally and dosing the aquarium is basically a waste.
As an aside, the fish I am currently treating all greedily ate the metro laced food but I should have mentioned that both the flake and pellets used are NLS Thera A with garlic, so perhaps the garlic masks the bitter metro.
I do not mean to be confrontational and am just indicating experiences I have gathered. Sorry, I did not intend to hijack this thread.
Scrounging around I can't really find many cases where infected fish will eat. Maybe the NLS is too tasty/ (I use it too)
From memory flagyl metabolises into a DNA destroying agent in susceptible organisms. I forget the mechanism but it's likely on the web.
If anti-flagellates are useless in the column then likely many treatments are a waste of money, and that may well be the case.
What you're saying is that flagyl will kill the trophozoite on the fish, presumable while it feeds on fish skin soup.
This obviously requires protist toxic levels to be in the fish epithelium for the short duration of that stage. This build-up would take time. Perhaps too long.
This would be a breakthrough to the aquaculture industry. Remembering you are killing the organism prior to the multiplication stage. Thus an opportunity for prophylaxis arises (and all the resistance issues)
The journals however seem silent on this. It would also mean the the free swimming theronts absorbs nothing from the column or at least not in the manner that counts, at least for flagyl.
I imagine a very easy experiment to do.
Just reading Some Seachem stuff. The issue with flagyl in the column seems related to solubility not uptake.
They recommend their PolyGuard with sulfathiazole.