You should change your carbon filter (the cartridge) every month. Carbon removes any odors from the water, the yellowness, and any medicine or chemicals that may get into the water.
It won't harm your fish if you don't change the carbon. I'm guessing, though, that your carbon is in a filter cartridge and those can get dirty. Not that being dirty will hurt your fish, but is the filter cartridge gets clogged, it cuts back on the amount of water being turned over in your tank, and in extreme cases, it can cause the filter to over flow.
You can not add gravel to your filter to help with filtration. Depending on your filter, you can add additional media if space allows. Some filters come with baskets to add additional media - like more carbon, zeo-lites, filter floss, bio-balls, lava rocks, etc. If there is space, I would add something for bacteria, like the bio balls.
As for what other things you can do to help your filter, filter your tank - make sure you are not over feeding. Keep up on water changes. Make sure the impeller doesn't get funky. Rinse the dirty filter cartridge off in fish tank water when you are doing water changes (if you rinse it with tap water, it will kill the bacteria).