Lucy and Carol are correct. I worked at a Petsmart and their filtration system was HUGE. Check out this one here. This is for a saltwater, but it shows the level of complexity that goes into commercial filtration systems. And using the man's hand as a reference, I believe that the main part of the filter (the grey tank there) is actually smaller than the one at the Petsmart I worked at. As for the overstocking, as Carol mentioned it is not meant as a permanent home so the fish will never have a chance to grow to their full size before being sold. Despite this, also keep in mind, many of the fish they keep in these overstocked tanks don't make it. Many of the stores suffer heavy losses on their fish, but the prices they pay to their suppliers compared to the prices we pay to them makes up the cost difference for the losses. Even those that survive are not always healthy. How many times have you walked into a fish store and seen tank after tank of ICH?