The internal filter is one of the simplest items out on the market. Mainly used in cold water and tropical freshwater tanks they make up a huge portion of sales for first time aquarists. They are completely submersible within the aquarium and draw water through the base (where the internal compartments are) by a powerhead that sits on top. The water is then shot out (similar to a powerhead) through a nozzel which can be attached to a venturi system.

A venturi system is a slip plastic tube raised above the surface. When water is forced out of the motor, the motion draws air through the venturi and out into the aquarium. This is a simle method of oxygenating the water, however most invertebrates tend not to like this method of airation, and close up on contact. There is normally very little room for filtration within the internal unit, and normally only contains a set of Large Particle and Fine Filter Sponges. This acts both as a mechanical and biological filtration unit, with the option of placing cardon packets within.

Some never systems have integrated a box within the system where bio balls or other porus material can be placed, this aids bacterial growth, thus imporving the filters performance. Reef systems however require a larger surface area then the internal system can provide for the bacterial growth, and a large box within your aquarium takes up valuable space. The maintenance of this system is very messy, as you have to lift the canister from the motor within the aquarium, thus releasing accumulated detritus.