I filled mine with filter floss. If you have extra ceramic biomedia I'd stuff it in there. You can also get an AC70 sponge and cut it into pieces that fit in there.
When I had one I filled the gray cartridges with carbon, look for carbon with larger pieces and rinse well to make sure the carbon doesn't fall through the openings otherwise it gets in the impeller and makes a lot of noise. If this happens pump the impeller housing up and down, the debris will generally work itself free. You can also stuff floss between the cartridge and the back of the filter which eliminates the need to buy the premanufactured filter cartridges.
thank you guys i put carbon in the grey cartridges big carbon and on the side where the water flows out i put a peace of filter cartridge so carbon cant come out