It depends on the type of zeolite as there are many different types. The most common used in aquaria are analcime, chabazite, phillipsite and clinoptilolite. Each zeolite absorbs different chemicals, zeolites can also be modified to absorb other chemicals or to improve its absorbance. The most commonly used zeolite in freshwater is clinoptilolite for the removal of ammonia. Analcime is used to absorb nitrite and nitrate. In saltwater zeolites act differently due to the high amounts of sodium. There are a few theories as to why they work so well with carbon dosing.