How does petsmart/petco keep their fresh water fish tanks so crystal clear?

Most of the multi-tank pet stores have an interlaced filtering system, where all the water flows into large filters and then is dumped back into each individual tank. They also constantly scrub the algae off the side walls. Whenever you change the filter cartridge, you will be removing bacteria with it, no way around that. However, lots of the bacteria live on the gravel and plants and decor, so you will still have an adequate number of colonies to take care of your tank. Just don't do a water change on the same day that you replace your filter media. If you do, you may experience a temporarily cloudy tank, as the bacteria re-colonize.  more