Congrats on the canister filter. I think you'll love it.
Like Peva said, replace the carbon. I do mine about every 4 to 6 weeks. I buy bulk, loose carbon and put it in a mesh bag. It's much cheaper than buying pre-packaged carbon.
A filte is for more than keeping your water clean. It's also removing harmful chemicals. The carbon adsorbs some, which is why we replace it. The other media encourages bacteria to grow that converts others into less harmful chemicals, that we remove via water changes.
Here's a great thread on canisters and media. Even if yours isn't listed, the general principle can be applied.
viewtopic.php?f=9&t=30145