The substrate is fine as long as it's recommended for aquarium use. The chemical medium or carbon is fine to use too. It will help filter the water and reduce odors. I would use it according to the instructions and when it runs out, don't buy any more.
Once the tank is cycled and running for a time, you'll need to remove and replace a percentage of the tank water to maintain safe water conditions for your fish. The more you change and the more often you change it the healthier the fish and plants.
If you do large, frequent water changes, you don't need to use the carbon, the water change does the same job.
"Fear not, my young apprentice. Just change the tank water."