Water flows through the gravel, under the plate, then pumped up the up-lift tubes that's mounted onto the plate in the back of the aquarium. Water is then pumped via an air-pump or powerheads. The tops of the tube, or powerheads, should be positioned so that water coming out is pushed across the top of the water. Surface movement is how alot of the bad gases escape, and how oxygen is absorbed into the water. This is one of the better filter systems for a freshwater aquarium.
Water is pumped into the filter where it usually passes through a mechanical and a chemical filter.
Then the water overflows, it gives a waterfall effect, from the filter back into the aquarium. Some outside filters have a bio-wheel mounted on it for better bacteria growth. This also is one of the better filter systems for a freshwater aquarium. Will give a better biological rating if equipped with a bio-wheel. This is the best filter to use on an aquarium with oscars, and other gravel moving fish.