Natural Pond Handbook

07 · Filtration Systems

In a natural pond, "filtration" is mostly biology. The goal is to grow the right microbes and plants, and to keep solids from rotting in the water.

Mechanical filtration

Removes physical debris before it decays:

  • Skimmer box — draws floating leaves/surface film into a basket.
  • Settlement chamber — slows water so particles sink and can be drained.
  • Brush / mat media — traps fines; must be cleaned regularly.

Biological filtration

The heart of a natural pond. Water passes over media (bio-balls, lava rock, Kaldnes, or simply gravel and planted beds) where bacteria colonize and convert ammonia → nitrate.

A shallow, gravel-filled planted bed plumbed so pond water is pumped down through the gravel and up through the plant roots. This is the single most effective natural filtration method:

  • Roots + gravel = enormous bacterial surface area.
  • Plants drink the nitrates.
  • Water returns crystal clear and oxygen-rich.
Best practice Size the bog to ~10–30% of pond volume flowing through it continuously. A pond with a proper bog filter often needs no chemicals and no UV to stay clear.

UV clarifiers — use or skip?

A UV clarifier kills suspended algae (green water) but does nothing for the underlying nutrient excess, and it also nukes beneficial plankton. In a true natural pond, aim to avoid needing one via plants and balance. Keep it as a last-resort tool for stubborn green water.

Constructed wetland / reed bed

For larger or swimming ponds, a separate reed bed (Phragmites, Typha, Schoenoplectus) polishes water through dense root systems. Water enters one end, trickles through gravel and roots, and exits clean.

Filter sizing rule of thumb Your filter must handle the full pond volume every 1–2 hours. For a 5,000-litre pond, you need a pump rated at 2,500–5,000 l/h, and a filter rated for at least 5,000 litres.
Biological vs mechanical: you need both

Mechanical (sponges, foam, mats) catches debris. Clean it every 2–4 weeks by rinsing in pond water — never tap water.

Biological (bio-balls, K1 media, gravel) houses beneficial bacteria. Never clean it thoroughly — a gentle rinse in pond water is enough.

UVC clarification If your pond gets green water despite good filtration, a UVC clarifier will clear it in 3–7 days. Size the UVC for your pond volume, not your pump flow rate.