Velvet is a death sentence - there’s no trying to build immunity to that, and it’s the primary reason hobbiest QT.
It truly is “Russian roulette” when it comes to velvet. No amount of prevention inspecting will protect your tank from that.
I don't actually believe this 100%. OK, not even 50%.

It is very possible to keep your fish immune to seemingly everything as my tank proves. Yes, I did smoke for a number of years and so far, don't have cancer that I know about, But I think the Agent Orange I am full of may be suppressing the cancer to give me something worse.
I feel and it's just my opinion, that if your set up your tank correctly, and feed correctly you can have an immune tank.
Feeding correctly is something that is rarely if ever done in this hobby. Feeding correctly to keep fish "immune" IMO, relies on getting living gut bacteria into your fish. That means not relying 100% on store bought foods like frozen. Gut bacteria controls the immune system of a fish and us.
My reef is 54 years old and I never quarantined anything, not even my 30+ year old fish. Nothing has ever died of a "communicable" disease in about 45 or so years and I add fish from everywhere including the sea.
I also believe, and I'm sure I am the only one, that quarantine and medication is counter productive and severly shortens the life span of fish and almost completely destroys the immune system. But for a new tank owned by a Noob, they may have to quarantine because you need an aged system and some experience to keep fish in the state of health needed for this natural system.
If our fish are not almost always dying of old age, we failed. Also all healthy, paired fish that can spawn in a tank, should be spawning as that is the best indicator of health as healthy fish constantly spawn, like every couple of weeks.
Velvet or any other disease is a non-issue for me so I must be very lucky. I may just go and ask Scarlett Johanson for her phone number.
