Like bk828 said all of the large chain stores and even some of the mom and pop lfs have a large sump system that filters water from all the tanks. What this means to the store is that a water change can be a huge water change for the system as a whole and not needing to mess with each tank. It also means only needing to maintain the one large filter rather than lots of smaller ones. What it means to us is that any disease that is carried by the water is in every tank. If you see ich in one, every fish in the store should be seen as having ich until proven otherwise.