Mechanical Filters – Skimmers

Determining the best skimmer for a particular project and how best to use it can be a challenge. Choices for skimmers come from both the water garden industry and the pool industry. The use of a pond skimmer started with water gardening as a way of moving the submersible pump out of the pond and into a protected space, away from the fish and plant debris. This created a form of pre-filtration with leaf nets and Japanese matting to separate out the larger debris ahead of the pump.

Skimmer Basics

The primary job of a skimmer is to pull the large, floating debris off the surface of a pond before it has a chance to saturate and settle to the pond bottom. Ponds also generate a film on the surface created by oils from plant debris, fish, food, dissolved organics, et cetera. Even a pond that is completely protected from the elements can produce this film, creating a dingy look to the water’s surface. A properly functioning skimmer pulls this film off, keeping the surface clear.

An example of a weir operation.

A skimmer works with the use of a weir: a floating door that pivots at the bottom with a top edge that is suspended just below the surface of the water. The incoming water is forced over the top edge, creating a high level of tension at the water’s surface. The closer the weir edge is to the surface, the better it will create this surface tension. There are many types of weirs, but they’re all designed around the same goal: removing surface debris.

Many water garden skimmers create poor surface tension and have poor skimming ability due to a weir with little buoyancy or side channels that open up to the skimmer’s interior as the weir opens with water flow. In “skimmer only” ponds that don’t incorporate other water outflow types like bottom grids, bottom drains or mid-