An external power filter is a sealed canister under the aquarium. The water is run through the filter media inside the canister and is pumped back into the aquarium. This water is then colonised with bacteria which reconverts the water into a safe habitat.
An internal power filter is fitted inside the aquarium. The base of the filter consists of small slots and grids. Water is pulled through this media through a pump and is then pumped out through the top of the filter. This water is colonised with bacteria to reconvert it into a safe habitat.