With activated carbon, each piece of carbon is like a little sponge that traps odors, medication residue, and dissolved fish waste. Carbon also removes discoloration in cloudy water. Activated carbon will no longer effectively absorb anything and should be replaced after about 3 or 4 weeks. To test the absorption power of carbon, take a glass of aquarium water on white paper. If the paper looks yellow through the glass, it’s time to change the carbon.

An external power filter hangs on the back of your aquarium and is basically an electric pump that draws water from your aquarium and pumps it through a replaceable filter cartridge that is typically filled with activated carbon. The “carbon cartridge” traps any floating debris such as excess food, and provides the chemical and mechanical filtration. Biological filtration is accomplished by passing the water over a wheel, sponge or porous plastic plate that houses the beneficial bacteria.

In some cases the beneficial bacteria live on the replaceable cartridge. You should only use a filter designed in this way with an under gravel filter as other biological filters since replacement of these cartridges removes the beneficial bacteria from your system.

Power filters come in different sizes and are rated by the number of gallons per hour that they can pump. This means the water should be circulated through the power filter a certain number of times per hour. It is recommended that the aquarium water be circulated through the power filter three to five times per hour.

For example, if you have a 20-gallon aquarium, you’ll need a power filter that circulates up to 100 gallons per hour. For a 55-gallon aquarium, a power filter that circulates up to 300 gallons per hour would work best.

Most power filters are sold with at least one filter cartridge included. Some cartridges are reusable. They can be rinsed clean and the inside of the cartridge refilled with fresh carbon and/or filtering media, but most cartridges need to have the filter cartridge replaced on a regular basis. If you buy a filter that has reusable cartridges, be sure to rinse the cartridge in some of your aquarium water or in dechlorinated water. Chlorine will kill the beneficial bacteria that have developed on the filter cartridge. We suggest that the filter cartridge or media be changed at least every three to five weeks depending on the fish load and type of fish you are keeping.

Remember, when picking the filtration for your aquarium, you need 3 types of filtration — mechanical, biological and chemical, all working together to have the happiest and healthiest fish.