Interesting, I've been researching this in depth for my new C3 and you are both confirming what others are saying all over the net. I already have the Seachem Purigen 100ml in Filter Bag in my "cart" at Amazon !!!!!!! I hope it fits !!
Further, it is interesting that Fluval's AquaClear 50 uses a reusable sponge whereas Fluval C3 uses a replacement poly foam cartridge. So in addition to making them more money on replacements, you can lose a lot of beneficial bacteria when the cartridge is replaced rather than washed !!! Thus putting in a piece of custom cut aqua pure filter media (or equivalent) and changing at differing intervals sure makes sense.