PetDumb

I bought an under gravel filter system for my aquarium from Petco. It was supposedly a Petco product, as that was the brand. Inside there are cartridges with activated charcoal that are supposed to be replaced monthly (I think they recommend every two-weeks). I went to Petco to get the replacement filters and they told me they don't have any! I was also told the under gravel filter systems have gone on clearance—though they are still selling them. Thanks to the Internet I found that PetSmart sells the (Top fin brand) replacement cartridges that will work in the Petco system.
So why would you sell something (and continue to sell it) that needs replacement parts—but not sell those parts? That is just dumb!