Natural Pond Handbook

35 · Koi Ponds

Illustration of koi in a pond
Koi need depth, a bottom drain, and serious filtration.

Koi are large, messy, long-lived fish. A goldfish pond will not do — koi demand depth, filtration, and discipline.

Koi vs goldfish

Koi grow to 30–60 cm and can live 30+ years. They uproot plants, stir sludge, and produce far more waste. Treat them as the pond's purpose, not an afterthought.

Pond specifications

  • Depth: 1.2–1.5 m minimum, deeper in hot climates — koi need cool, stable water.
  • Volume: 5,000 L+ for a few fish; allow ~1,000 L per mature koi.
  • Shape: rounded, no sharp corners where fish can scrape.
  • Bottom drain: central or multiple, to pull sludge to filtration.

Filtration

Koi need serious mechanical + biological filtration: a bottom drain feeds a vortex or sieve (pre-settles solids), then a moving-bed or bead filter for bio-treatment, often with a large secondary bog for polishing. Turn the total volume over every 1–2 hours.

Stocking, feeding, quarantine

  • Start with few fish; overstocking is the #1 cause of sick koi.
  • Feed a quality pellet; fast fish below ~10°C.
  • Quarantine all new koi 2–4 weeks in a separate tank before introduction.
  • Net against herons and raccoons; consider a pond tunnel or scarers.
Costs A proper koi pond with filtration typically costs several times a wildlife pond of equal area. Budget filtration and a reliable pump before fish.
Koi pond requirements
  • Minimum volume: 10,000 litres (ideally 15,000+).
  • Minimum depth: 1.2 m (deeper in cold climates).
  • Filtration: Must handle full volume every hour. Biological filter volume = 10–15% of pond volume.
  • Flow rate: Pump should circulate 1–2 times per hour.
  • UVC: Essential for koi ponds — koi produce high waste loads.
Koi grow large A 150 mm juvenile koi will reach 600–900 mm in 5–8 years. Do not stock a small pond with koi — you will be forced to upgrade or rehome them.