The most common chemical filter media used is activated carbon or charcoal. Activated carbon absorbs many pollutants that are in the water column. Depending on the filter you use, carbon can be included in filter cartridges, or carbon media bags.
Activated carbon is relatively cheap, and can easily be purchased. There are many different brands on the market, but most do the exact same thing which is Absorb impurities.
Many tropical fish keepers do not use activated carbon on a regular basis, but rather only when needed.
So, you may be asking yourself, well when would I need to use activated carbon?
Activated carbon can be used when you first set up your tropical fish tank. It is a safe practice to run the activated carbon in your filter for several weeks to ensure that any contaminants that may have been on the aquarium glass, decorations, or substrate are removed from the water column.
One may also use carbon to remove discolorations in the water. If you use driftwood in your aquarium tank, you may notice that your water has a tint of brown to it, this is called tanning. Activated carbon will absorb the tannins and help clear up the water.
Another use for activated carbon is that it removes medication from your water. If you ever have the misfortune of medicating your tank due to a sick fish, activated carbon will remove the medication from the water column.
Lastly, activated carbon will remove pollutants that cause odor in your aquarium water. If you have a “funky” smell to your aquarium, there is a reason. This reason could be many things including decaying food, a dead fish, or just poor water maintenance.
Activated carbon will only last 2-3 months in your aquarium. After this time, the carbon needs to disposed of and replaced.
Using activated carbon will help, but I must state that most of the benefits of activated carbon can be matched with regular water changes. Do not depend on carbon alone.
Ion Exchange Resins
There are two types of media that apply ion exchange principles. Man-made and natural resins.
Man-made resins and natural resins attract contaminating pollutants and exchange those compounds with beneficial compounds. Ion exchange resins work great in combination with activated carbon.
Synthetic Polymers are very similar to activated carbon. The exception is that synthetic polymers can reduce ammonia, nitrite, and nitrate. Synthetic polymers can also be recharged and reused many times over and over again. Unlike activated carbon that only last a few months and then has to be disposed of and replaced.
Phosphate controlling media
This type of chemical filter media absorbs phosphates and other pollutants from your water. High Phosphate levels are a contributor to algae growth. If you aquarium has a very bad algae problem, phosphate-controlling media could be used along with regular water changes to help control this problem.