Cartridges are usually blue or black plastic with a sponge attached to one side, with the carbon on the inside. Some filters use a little bag with carbon. There's also a black carbon sponge. The plastic cartriges should be replaced every 4-6ish weeks, sometimes longer depending on how well they hold up. I've never used the bags of carbon, so I don't know how long those last for. The black carbon sponges just need a gentle rinse in tank water during a water change and put back into the filter, they can last for a very long time and will hold a lot of the good bacteria your tank needs.
Edit: looking at google, it looks like the black carbon sponges will occasionally need replacing, and may not hold as much of the good bacteria as I thought.
It looks like you have the plastic cartridges. They do need replacing fairly regularly, though a lot of people will rinse them in tank water and reuse longer than the manufacturer states. I've done that and had no issues, just waiting until it's falling apart.
Do you know what kind of filter you have?
Last edited by styggian; 01-31-2012 at 03:53 PM.