My opinions: (And I am sure others will vary a lot)
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Try not to use a really tiny tank!
First, it has a big impact on water quality.
You cannot rely on bio-filtration that much in a QT, especially if you medicate.
So you need enough volume to dilute ammonia until you change the water.
Especially if you are the type to occasionally miss a day of water changes. (Like me)
Second, a small tank can further stress a really freaked out fish.
And that seems to matter a lot.
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I have given up on long term medications. For me, I have never seen any real difference.
Some fish with ich get better. Some die. Source pet store, medication, etc, does not seem to matter.
The only thing that really seems to matter is water quality.
If you forget to check/change water enough - bad things happen.
That said, anecdotal observations are hardly scientific experiments.
** I still do dips. Especially when I first get a fish/coral.
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My current QT setup is rather interesting. Big, and it is actually plumbed into the main system.
Return from QT to main tank goes through a 5-micron then a 1-micron filter.
Not without risk, especially viral, but small enough to stop ich.
Key is that water quality is great, since I essentially replace several gallons per hour.
Plus no need to acclimate when transferring at the end.
And it can be disconnected if I really need to.