Chemical filtration is the use of chemical process to filter the water. It can be in the form of solutions that bind particles together so that they can be picked up by the mechanical filter, or in solid form such as activated carbon and zeolite. Chemical filtration have a 'limit' of how much they can filter. When they are "full", they need to be replaced.
Some common chemical filtration is
Chemical filtration should only be used in emergencies and not as a permanent filtration that you keep replacing regularly.
Chemical filteration and their uses:
Activated carbon can be thought of like a vacume cleaner, taking in everything that it comes across. It is useful for removing medications from the water after you cure your fish and you do not want the medication to linger in the water. (Medications is generally harmful to aquarium plants and the biological filter, do not use medications unless absolutely necessary! You should cure your fish in a separate hospital tank whenever possible. In extreme cases where you have to medicate your main tank, always remove medicine traces after your fish are cured! Learn about curing fish diseases here)
Other than removing medications, it is also useful in removing colour caused by driftwoods or dried leaves etc.
Activatvated carbon also removes ordours. However, note that a healthy aquarium should not have ordours! Don't treat the symptom without treating the source!
Zeolite and other ammonia removal chemicals
Zeolite is used to remove ammonia from water. There are also other chemicals that removes ammonia from aquariums, however for this section we use zeolite to represent zeolite as well as those other ammonia removal chemicals so this will be more readable..
As said earlier, zeolite should not be used as a permanent solution to your aquarium filtration even if you replace the zeolite regularly when they get 'full'. They should only be used in temporary installations where you don't biological filteration in place, for example temporary feeder fish tanks, quarrantine tanks, hospital tanks etc. You don't want to have bio filteration is these tanks because you often need to medicate these tanks. When you medicate, you'll destroy the bacteria colonies and wreck your bio filter anyway. And being temporary tanks, the bacteria colonies does not have any supply of waste chemicals to sustain the bacteria colonies when their fish are moved into your main tanks.
When you use zeolite, you are artificially removing ammonia (which reduces nitrites as well since nitrites is converted from ammonia). As a result, you 'trick' the biological filter into thinking that you have a smaller bioload than you actually have. As a result, your beneficial bacteria colonies downsizes as the 'excess' bacteria staves and dies.
While this is fine while your zeolite is hungrily absorbing ammonia, what happens when they are full and will not absorb anymore? When this happens, your beneficial bacteria will have to slowly multiply to increase their capacity to process the suddenyl increased ammonia and nitrites. Before they have multiplied sufficiently, your aquarium will have these excess waste chemicals in the water. How much a spike it will cause depends on how much zeolite you were using.
Instead of replacing zeolite and wasting your money and time, why not let your beneficial bacteria handle it for free?