Throw in a Purigen pouch instead of carbon. Should fit your AC 50 noooo problem! Pull it out from time to time when it gets brown, throw it in a 1:1 bleach:water solution for 24 hours, rinse, soak in heavily Primed water for a few hours(SeaChem says 24, they also say to use far too much Prime) and then back in the filter it goes.I wanted to do my water change every week, so, carbon might be useless most of the time. What is better to replace the carbon in my filter? More mechanical filter, like sponge, foam or pad, or more bio filter?
The only piece of media I EVER replace is my filter floss pads when they start breaking down. Only run carbon pads after meds, you'd also need for unwanted chemical exposure in the tank....that's never happened to me though.