Activated charcoal provides important chemical filtration of aquarium water, removing harmful toxins such as chlorine and heavy metals that sicken or kill pet fish. Over time, however, charcoal reaches it limits of what it can absorb, making necessary the cleaning or replacing of the carbon. Proper preparation--and knowing how to use your ears when removing the charcoal--are key to successful cleaning or replacing of activated carbon in aquarium filters.
Remove the filter from the tank if possible after turning off the electrical supply and shutting off any water shut-off valves depending on filter type.
Carry the filter over to a sink or tub, moving carefully so no dirty aquarium water spills out onto carpets, rugs or other flooring.
Remove the loose charcoal or charcoal cartridge from the filter, depending on filter type.
Place the carbon in the mesh kitchen strainer and run warm water through the charcoal to rinse out impurities if using loose charcoal. If using charcoal cartridges, allow warm tap water to run through the cartridge for several minutes.
Hold the rinsed charcoal up near your ear. If you hear a sort of subtle crackling or popping noise emanating from the charcoal, place it back into the filter. If you hear nothing, the charcoal is no longer activated and should be replaced with fresh carbon.
Clean or replace activated charcoal at least every month to maintain consistent chemical filtration in aquariums.
Never rely on activated charcoal as the sole filtration element in aquarium filters. It only removes some harmful chemicals and toxins, not large particulate matter, and charcoal performs no real biological filtration, an essential method that grows beneficial bacteria helpful to fish immune systems. Combining charcoal with mechanical filtration sponges and ceramic rings or cylinders that promote beneficial bacteria growth is the best route.