Lfs And Qt Their Fish

[nmhs2]

Ok so i got a question on LFS's. Say they get their shipment of fish in and acclimate them into the tanks. Those fish stay in there at least days to weeks maybe months. Anways what brings them into such luck with their fish not getting ich through their tanks? Think of it its like one big QT right? What if one fish has ich, most setups are interconnected through all their tanks, what makes that ich not spread to other fish, or possibly questionig QT process?