Zeolite is a natural stone that has the properties to bind nitrates and phosphates. Zeolite is an important filter medium to keep the pond water healthy and algae free. Renew the Zeolite once per season to keep the pond water in optimal condition.
Its main use is to absorb ammonia in pond water that the fish produce, its always recommended to keep some as spare incase of an ammona spike in the summer periods.
Remember to remove this from your filter when a chemical is added to the pond, such as any parasite treatment. This loose substrate should be used in a filter media bag, and is then easy to replace as needed.