I've gone through many changes. Since starting in the late 70's I never bothered with QT, only a "hospital" tank when needed. On my last build though, with so many issues for many lately, I planned a strict QT. I did TTM with two rounds of Prazi, then observation. Then along came a regal angel. "Looked" ok, but wouldn't eat even once in observation. Hard to see velvet, if not bad, on a fish of that color. My wife convinced me maybe it would eat if in the DT, more spots to hide, more comfortable, etc. BANG, hello velvet and a fallow period after losing 3/4 of my fish
@Humblefish calmed me down and helped me through emergency options, and I was also lucky in obtaining a prescription for CP, and it's my new best friend. However, my current QT is pretty crazy for most people. I had already had issues with many fish arriving and if I treated for bacterial issues they died of velvet, if I treated for velvet they grew fuzzy patches and dead quick too. Drove me nuts. I have the space throughout the house though, plenty of tanks, and a mixing station, so I got creative. Very aggressive, likely stressful, but has worked.
New fish go into a QT for 24 hours with methelyn blue and room lights out to help them settle and relieve any ammonia burn issues. This is followed by a modified TTM with tank transfers every 48 hours. NOT actually treating them for ich, but 48 hours lines up with antibiotics so I transfer instead of water changes and start off with Metroplex and Kanaplex for all for 10 days. Two of those tank with General Cure (contains metro as well anyway) instead of just metro. It hasn't seemed to stress the fish any more than sticking a siphon in and messing with a water change. My "thinking", and has appeared to be working, is the TTM part of it is at least clearing up any ich, but also holding velvet at bay with the frequent tank transfers while I worry about infections. Then, into a 29g they go for 14 days in CP, then to another 29g to finally just observe.
Actually, the final 29g is near my parrot, who now says "Good morning fishy", lol
Just an example of finding what works for you! I have plenty of water, space, and tanks so this works. Others do better with longer in copper and observe.