Types of pond
filtration systems.

§01 — The three stages, in order.

Outdoor pond filtration is typically described in three functional stages. Any given piece of equipment may perform one stage, two, or all three. The stages, in the order water passes through them, are mechanical (removing suspended particulate), biological (converting dissolved ammonia to nitrate via nitrifying bacteria colonies), and clarification (reducing free-floating algae, most commonly using ultraviolet light).

Most pond-filter product listings describe themselves in terms of which of these stages the unit performs and for what pond volume the manufacturer rates it. A single-stage mechanical filter and a three-stage combined unit are both legitimate products; they are simply sold for different applications.

§02 — Mechanical filters.

Mechanical filtration removes solid particulate from the water column — leaves, silt, uneaten food, waste. The most common forms are foam cartridge boxes, pleated-cartridge pressure canisters, brush arrays, and sieve screens.

• Foam cartridges. Most common mechanical medium. Requires regular rinsing.
• Brushes. Appear in first-stage gravity-fed chambers; capture coarse debris with low pressure loss.
• Sieve screens. Self-cleaning static wedge-wire screens used upstream of larger systems.

§03 — Biological filters.

Biological filtration provides a high-surface-area substrate where nitrifying bacteria establish colonies. Two bacterial groups are involved: Nitrosomonas, which oxidise ammonia to nitrite, and Nitrospira, which oxidise nitrite to nitrate. Both require dissolved oxygen and a stable surface to adhere to.

Common biological media include rigid plastic matrix blocks, loose ceramic noodles, sintered glass, and moving-bed plastic elements such as OASE's Hel-X, which stay suspended by aeration and self-clean through collision.

§04 — UV clarifiers.

A UV clarifier is a chamber through which pond water is pumped past a UV-C lamp. Single-celled algae suspended in the water receive a germicidal UV dose on each pass and aggregate into larger clumps that a mechanical filter can then remove. UV clarifiers address green-water turbidity specifically; they do not affect filamentous string algae on pond surfaces.

Sizing is expressed in watts per gallon of pond volume, with manufacturer ratings typically in the range of 1 W per 100 gal to 1 W per 250 gal depending on target clarity.

§05 — Combined / all-in-one units.

Many products sold for small and mid-size ponds combine two or three of the stages in a single housing — a pressurised canister with foam prefilter, biological matrix, and an integrated UV lamp is a common configuration. Larger installations tend to separate the stages into discrete components plumbed in series.

§06 — Glossary.

• Turnover. The time required to pump the pond's full volume through the filtration system once. Usually expressed in hours.
• Head pressure. The vertical height water is lifted by the pump, which reduces effective flow rate.
• GPH. Gallons per hour — the standard unit for pond pump flow ratings.
• Gravity-fed filter. A filter that receives water at atmospheric pressure from a bottom drain, rather than being plumbed inline with a pressurised pump.
• Moving-bed media. Buoyant plastic elements that tumble in an aerated chamber, self-cleaning through collision.
• Nitrification. The biological process converting ammonia → nitrite → nitrate, performed by two bacterial groups.