Unfortunately, the overwhelming majority of LFS and retailers have velvet and ich, and who knows what else in their systems -- this because of the sheer volume of fish that have been in and out of their systems, and the fact that they cannot keep fish in full therapeutic levels of copper for a full thirty days before adding new to a system (they're usually one to three systems and dozens of tanks). It's not economically feesible for them to properly treat an qt fish. So, as the end user, we need to take it upon ourselves to protect and treat them. Low copper levels in LFS and Distributors will mask obvious symptoms, and occasionally paint the illusion of a parasite-free fish.
Unfortunately, there's nearly 100% chance a fish you buy these days is afflicted with ich, velvet, brook, flukes, infections, or intestinal parasites and often several of these at the same time. This was always the case but the supply chain was under heavy pressure to lower prices and the fish are suffering. We can say we don't like it but we are almost ALL cheap and to some degree responsible. Velvet has also become exponentially more common that it used to be, every bit as common as ich IMO, IME.
So, cheaper fish but their survival rates are often more abysmal, and now the pressure is on us to properly quarantine and preventatively treat our incoming fish. Proper quarantine and treatment isn't risky, it's a lack of knowledge that causes QT deaths. I have had exponentially better success with ALL fish since abandoning "ich management" techniques and treating everyone before they go in to the DT.