The idea behind chemical filtration is to strip toxins out of the water. Activated carbon and zeolite are commonly used to do this. They both have tiny pores that capture the Ammonia when water flows through them. When activated carbon fills up with Ammonia, you have to replace it. If you don't, Ammonia will start to leech out back into the water. When zeolite gets filled up with Ammonia, it can be recharged by letting it sit in a bucket of salt water for a day.
Activated carbon and zeolite are good choices if you have a small pond with only a few fish in it. If you have a bunch of fish, the costs of replacing activated carbon will add up over time. Using zeolite is a good option, but the more you have, the more you will have to heave around to clean.