Been a "lurker" for a while, but this is my first post. I have the Top Fin 5 gallon tank that many members on this thread have and need to change the filter cartridge. I have been to Petsmart twice now and both times they have sold me the wrong filter cartridge. The one that is currently in there is the one that came with the tank when I bought it about 3 weeks ago (it is blue and slides in). They keep insisting that I need a tetra biobag, but that won't fit in my tank. When I told them this they told me that I must not have a Top Fin tank then, but I still have the instructions and that is the name on the instructions. If anyone knows the name/ size of the correct replacement that I need I would greatly appriciate it.