I'm kind of new to quarantining (I've done it once in the last 8 years), I had Chromis in a the QT for longer than month while setting up my newer and bigger tank. I didn't treat with anything, just watched them, they were fine for a long time, once in the DT they starting picking each other off, one by one until I had 1 left. No illness as far as I could see.
I had another long time fish that was in DT that I pulled out to quarantine, no idea what was wrong with him, no visible signs of anything, I dosed with copper only once, the fish died within the week. All other fish were fine.
I don't really know a lot about what to look for, other than the obvious, white spots, stringy poop, flashing. What is this about swimming into the powerhead flow?
I prefer to watch instead of treat.
Also, is there a better option that copper? It ruins the QT for all future snails etc. that I would like to quarantine. I just broke down my other tank because of the one dose of copper..which I stupidly forgot about when I bought 5 scarlet hermit crabs and dropped them in there 2 or 3 months later...they all died and after a few days I remembered why

and felt awful!
I see people saying velvet is a big problem these days, now I'm afraid to buy more fish once my QT is ready to go, I plan on getting a larger cuc for my big tank and at least one or 2 will go into the biocube. I'm also getting 2 tangs when the QT is cycled. I planned on observing them for at least month, is this enough? What should I watch for? what should I have on hand just in case? I can easily feed healthy immune boosting diet, any suggestions on an easy way to do that too? I feed NLS, I know it has garlic but I also know most prefer frozen food over dry.
For years I would just acclimate and drop a new fish into the tank without issues, now I'm over thinking the entire process! I've had the same 2 clown fish for 8 years, never had any issues with them, they were my starter fish.