Many aquarists use chemical media to remove toxins and alter water chemistry. If used, where should chemical media be placed? Generally, chemical media is placed at the bottom of the filter. If chemical media is placed first, it will become clogged and have to be replaced long before its chemical filtering properties are exhausted.
Placing chemical media before the biological media means that some of what the bacteria live on will be removed before reaching the biological colonies.
As a result, ammonia and nitrite loving bacteria will be less likely to thrive in the biological filter media. Placing chemical filtration media as the last stage of the filter ensures it does not affect the bacterial colonies. Types of chemical filtration media include activated carbon, zeolite, peat, ferric hydroxide, and various resins.