A surface skimmer is a really useful addition to any external filter

A surface skimmer (JBL TopClean II) is a really useful addition to any external filter, as it keeps the water surface clean and ensures the water above the surface is ideally enriched with oxygen.

Surface Skimmer - Fluval USA

Aquascape Signature Series Pond Skimmers

Innovative and customizable aquarium surface skimmer rapidly removes surface debris, biofilm, and other contaminates from the aquarium surface. Increases oxygen in the water by removing surface film and other floating debris. Also aerates the entire tank by mixing air into the water as the surface water is pulled through the specially designed GuppyGuard.

Black Hole Skimmers: Aquarium Surface Skimmer (Filter Media & Pump Not Included)

CK777

External Canister Filter 396GPH with Surface Skimmer

Floating Surface Skimmer for Soft Wall Pools

What is a pond skimmer? - Webb's Water Gardens

Oberflächenabsauger Aquarium

JENECA External trailer filter with UVC lamp, JENECA Hang-on filters met UVC

Do I really need a protein skimmer for a reef/fish tank? 30 + 36 gallons?

The Fluval Aquarium Surface Skimmer draws water from the surface of the aquarium eliminating undesirable surface films. Since the Fluval Surface Skimmer doesn't contain metal parts, marine aquariums in particular that do not employ a surface overflow will substantially benefit from the implementation of the Fluval surface skimmer.