Filtration is one of the most important components of a successful aquarium. Many filters include charcoal to help remove unpleasant odors and colors in the tank. Activated charcoal filters last only as long as the charcoal can still absorb impurities from the water; then the need to be replaced.
Activated Carbon
The type of charcoal used to filter aquarium water is called activated carbon. It’s made of the same stuff that makes up pencil "lead"—graphite. It has an extremely irregular porous surface, making it act like a sponge for bad odors and trace elements in both water and air. Along with being used in the fish tank, activated carbon is commonly used to purify drinking water and remove bad smells from the air. It can even be a lifesaver for people or animals who have been poisoned or have taken an overdose of drugs: when ingested, the carbon absorbs the toxic substance before the patient's body can.
How Long It Lasts
While activated charcoal is one of the most absorbent materials around, it can only absorb so much before it isn’t capable of taking in anything more. Charcoal filters will last between two and four weeks. The filter should be changed every few weeks to keep it active.
Potential Danger
Charcoal filters are generally regarded as safe, but some controversy surrounds their long-term use. Charcoal filters are great at sucking bad stuff out of the water, but they can also absorb trace minerals that are important for healthy fish and plants. Lack of certain nutrients in the water can lead to head and lateral line erosion, a condition also called hole-in-the-head disease that can be fatal to fish. Discus fish and oscars are particularly susceptible. Having a carbon filter doesn’t mean your fish will get the disease, but over-filtration of aquarium water can be a contributing factor. Charcoal will also suck up most medication, so remove this form of filter before medicating your tank.
Necessity
Charcoal filters aren’t necessary to a healthy aquarium. Regular water changes and biological filtration are enough to keep water chemistry in check. Carbon can be useful in the short term, such as to excess medicine from the water. Since it can get a little pricey to change out a carbon filter every two weeks, it's best to use this type of filter only in special circumstances, when your filter needs a little help getting the water clean.