Abstract: An externally-mounted aquarium filter assembly includes a tank which is supportable on a wall of an aquarium and partitions disposed in the tank for dividing the interior of the tank into a prefilter chamber and a post-filter chamber. The partitions include a pair of generally upstanding, spaced-apart first and second partitions, each of which has an opening therein to allow the passage of water from the prefilter chamber to the post-filter chamber and a raised partition base plate on which the partitions are supported, the partitions each defining an opening to establish communication between the chambers. A pair of filter cartridges are each independently removably mounted within one of the openings of the partitions so that water passing from the prefilter chamber to the post-filter chamber passes through the filter cartridges for trapping particulate matter in the water.