Koi Pond Filtration Calculator (Turnover, Pump GPH, UV Watts & Aeration)

How it works

load = 250 gal per adult koi + 40 gal per goldfish · filter flow = volume ÷ 1 h (koi) or 2 h (goldfish), × 1.25–1.5 when heavily stocked · pump rating = flow ÷ (1 − 0.035 × head ft) · bio filter ≈ 12% of volume (koi); media ≈ 1 ft³ per 8 g ammonia/day; ammonia ≈ food g × 0.35 × 0.10 · UV ≈ 6–15 W per 1,000 gal by sun · air ≈ 8 L/min per 1,000 gal + 1.5 per koi

A koi pond is an aquarium the size of a room, and its water stays clear and safe only if the whole volume passes through mechanical and biological filtration about once an hour, the biological stage has enough surface area to convert the ammonia the fish excrete from every ounce of food, and the water carries enough oxygen through warm nights. Pump flow must be chosen at the actual head, since lifting water to a filter or waterfall steals a large share of the rated output; ultraviolet sized to the pond's sun exposure keeps suspended algae from turning the water green; and the stocking rule of a couple of hundred gallons per adult koi is what makes the rest achievable.

Worked example

A 3,000-gallon pond with eight adult koi and ten goldfish is at 80% of capacity (2,400 gallons of fish load; room for one more koi). Koi call for a one-hour turnover boosted for the load — 3,750 GPH at the filter — so with 8 ft of head buy a pump rated near 5,200 GPH (≈ 250 W, about $29 a month). The bio-filter should hold about 360 gallons or 1.4 ft³ of moving-bed media for the 11 g of ammonia a day the normal summer feeding produces, with a 30 W UV in half-day sun and about 36 L/min of air.

Frequently asked questions