As someone who has spent a lot of time quarantining / treating fish over the years (& currently running a QT), I'd like to share some thoughts with the OP.
I use different methods to prophylactically treat MI depending on the species, whether its direct from a local collector or imported (along with an even greater diversity of parasites). I aim for very high water quality including very low nutrient levels, and this helps with minimising secondary infections due to treatment and stress, IME.
I use cupramine for most freshly caught local fish direct from collectors as they haven't been exposed to copper in all its incarnations for long periods of time. I don't like to ramp up, but agree it's easier on sensitive fish (and on their parasites, too!!). I use TTM for most sensitive fish, but have found that a fortnight is often inadequate. This is where having very few fish in a qt simultaneously is handy - it's easier to maintain high water quality, plus you're only dealing with one source of parasites. Don't mix fish from different sources in quarantine as you just end up dealing with more parasites and possibly infecting a fish that was previously pretty clean.
there are a lot of gray areas in successful outcomes. I am having less success with prazipro now for flukes, especially with imported fish. :facepalm: Formalin is effacious, but not for everything... Fresh water baths are effective, but again, not for everything. Finally each species and individual fish has a tipping point; a stage where it becomes the swimming dead. Even an otherwise hardy species can reach that point if it's been poorly handled through the chain of custody and especially once it's picked up a wide range of parasites along the way.
So your best bet is to be familiar with the species and it's likely intolerances, and the likelihood it's picked up additional parasites on its travels. Pick a proven treatment method, and do your best to carry it out properly. Finally, don't forget the six week observation period at the end. :beer:
HTH and GL
:wave: