It's been my experience over the past 30 yrs, that even the stores who claim to quarantine their fish, really don't. At least not in a proper quarantine process. That requires each fish to be isolated from all others, kept for a MINIMUM of 4 weeks in isolation, (even copper treatment needs 4 weeks) after which they can be placed in a community tank.
Most LFS who claim to quarantine, only keep the fish for a few days in holding tanks, usually with other fish, then if nothing apparent is going on, place them in the regular selling tanks...NOT a good quarantine procedure IMO...