Replacing filters on a regular basis is critical to the health of the plants, invertebrates, and fish livestock in your aquarium. Submersible filters are placed inside the aquarium. Most of these filters require an air pump and/or an air diffuser to work, and many have increased surface area for the cultivation of beneficial bacteria that free aquarium water from nitrogen and other contaminants. You should always have filters and filter replacements available for active and healthy aquarium environments.