Internal Power Filter Guide
How submersible internal power filters work, where they fit, and how to choose, position, and maintain them in small to medium aquariums.
What it is
An internal power filter is a submersible, motorized filter that sits inside the aquarium, typically attached to the glass with suction cups. An electric pump draws water through a media compartment and returns it to the tank, often through an adjustable outlet that also helps circulate the water. It is a compact, affordable option for small to medium aquariums, and it keeps all plumbing inside the tank rather than relying on external hoses.
How it works
The pump pulls tank water through a sponge or cartridge that traps debris (mechanical filtration) and provides surface area for nitrifying bacteria (biological filtration). The bacteria drive the nitrogen cycle, converting ammonia from waste to nitrite and then to less toxic nitrate. Many units accept additional media, including chemical media such as activated carbon to remove dissolved waste. The returned flow circulates the water and can be angled toward the surface to improve oxygen exchange. Internal filters are part of the broad family of submersible filters that also includes sponge and corner designs.
Pros and cons
- Pro: compact, self-contained, and inexpensive.
- Pro: combines filtration with adjustable water circulation.
- Con: occupies visible space inside the tank.
- Con: media capacity is limited compared with external canister filters.
- Con: must remain submerged, and the motor can be disturbed when removed for cleaning.
Choosing and sizing
Internal power filters suit small to medium aquariums, broadly from about 20 to 150 litres. The unit's rated flow should match the tank so that circulation is adequate without creating a current too strong for the inhabitants.
Installation
The filter is fixed inside the tank with suction cups, usually low on a side or rear wall, and must stay fully submerged while running. The outlet is aimed to spread flow across the tank; directing it toward the surface increases agitation and gas exchange.
Maintenance
The media sponge or cartridge is rinsed in tank water to clear trapped debris while preserving the bacterial colony; rinsing in chlorinated tap water should be avoided because it can kill the nitrifying bacteria the tank relies on. Because the unit lives in the tank, periodic cleaning of the impeller and intake slot keeps flow steady and prevents the motor from straining. Biological media should be rinsed gently rather than replaced wholesale, so the established colony is not lost. A common service interval for this filter type is around every three weeks.