I've been browsing the forum for a few days now trying to figure out exactly what people do for cartridge cores (other than buying new ones) I read the post about using stuffed animal fluff but that has a few issues of its own (yes i did try it) I saw quite a few references to fishtank filter filler but honestly the only thing i can think of that goes in a fishtank filter is the carbon and I'm sure that cant be what people mean though I cant find details on it. So I thought I'd ask and risk looking dense for being unable to find the answer on my own. What are people using and how?