Abstract: An internally-situated filter assembly for an aquarium tank includes an upright tank partition wall mounted within the aquarium for dividing the aquarium into an aquarium compartment and a filter compartment, an inlet formed in the partition wall generally adjacent to the top end thereof for allowing tank water in the aquarium compartment to flow into the filter compartment and an outlet formed in the partition wall for allowing filtered tank water to flow from the filter compartment into the aquarium compartment. A filter is mounted in the filter compartment generally beneath the inlet so that tank water may be fed thereto under the influence of gravity from the aquarium compartment and a catch basin is formed beneath and adjacent the bottom end of the filter compartment for collecting filtered water from the filter. A pump returns filtered water collected in the catch basin to the aquarium compartment. The filter assembly is especially intended for small salt water reef tanks.