The North petco, I assume. When they started up all their fish were sick.
Each column of tanks is on the same sump/filter, which is why they take them out of order the way they do.
The south petco isn't any better, and I think more of their tanks are connected than the north petco. I've walked in when they've had half a tank of dead guppies and noone noticed until I grabbed a manager and made him quarrantine the tank.
There is a reason I preferentially buy fish from The Reef in the Sears Mall.