To replace an external power aquarium filter, first open the filter cover to access the carbon cartridge. Slide the cartridge out of the filter and dispose of it, and fit a new cartridge into place. An undergravel filter typically does not need replacement; for this type of filter, regularly clean the tank gravel and change 10 to 15 percent of the tank water once a week for optimal filtration.
When replacing an aquarium filter, only replace one part of the filter media at a time. This helps beneficial bacteria to continue to colonize the tank, and avoids the need to newly cycle the filter. During filter cycling, fish may die because not enough bacteria is left in the habitat. Change carbon filters about every six months, depending on the size of the tank and the number of fish. Every two months, rinse dirty carbon cartridges and put them back into the filter to ensure that no harm comes to the bacteria, which help keep fish healthy by processing their waste.
If an undergravel filter requires carbon cartridges, change them three times per month. Always remove carbon filter media from a tank while medicating fish. Otherwise, the carbon may absorb the medication intended to treat the fish.
Rinse any reusable filter media in tank water before returning it to the filter, and do not use soap or hot water to clean the elements. This protects bacteria and also prevents chemicals from tap water or other sources from entering the system. The filter's sponge media can last for a decade before needing replacement; rinse it regularly to keep the filter from becoming obstructed.