Chemical filtration is the use of products to help remove noxious substances from the water.
An example of this is the use of Granulated Activated Carbon (GAC) within a filter chamber. Some filtration units are manufactured and supplied with a carbon filtration layer included. You can also buy activated carbon to add to your own filter’s media. Simple sponge filters used in smaller fishtanks do not have a carbon element.
It is important to remember to remove the carbon from your filter temporarily if it becomes necessary to treat your tank with medication, as the carbon will immediately filter out the medicine, rendering your attempts at first aid useless!