Koi pond equipment2026-10-03

How to size a koi pond filter, UV and air pump

Size all three from the pond's volume in gallons and how heavily it is stocked. By the most repeated rules, the filter should pass the whole pond once an hour at light stocking and twice an hour at heavy stocking. The UV light needs about 10 watts per 1,000 gallons to clear green water, or 30 watts to kill bacteria and parasites, and any flow beyond its rating goes around it through a bypass. The air pump should deliver about 28 liters a minute per 1,000 gallons at light stocking and 60 to 80 at heavy, through one diffuser for every 2,000 gallons. Each rule comes from a different retailer page, so the table below puts them side by side.

What the manufacturers and retailers say

No extension service or engineering body publishes these sizing rules. They come from manufacturers rating their own equipment and from koi retailers. Play It Koi, whose guides cover all three pieces, refuses automated fetches, so its figures are quoted from search snippets.

Filter

Play It Koi's bead filter guide (snippet) sets "1x turnover/hour" as the "Minimum for lightly stocked koi ponds (fewer than 1 inch of fish per 10 gallons)", 1.5 times an hour for moderate stocking, and "2x turnover/hour" for "heavily stocked ponds, show koi collections, or ponds with heavy feeding schedules." Aquadyne, a bead filter maker, rates each vessel by gallons, pounds of fish and pump flow. The AD1000 is rated "Up to 1000 gallons" with a "30lbs" fish load and a "500 - 3000 GPH" pump (123 Ponds). Webb's Water Gardens lists the AD4000 at 4,000 gallons, 85 pounds and 1,000 to 4,000 GPH, and the AD8000 at 8,000 gallons, 165 pounds and 1,000 to 6,000 GPH. The largest, the 8.8C, takes "1000 to 8000 Gallons Per Hour" (Underwater Warehouse). The larger vessels work out to about 2 pounds of fish per 100 gallons, which is the ceiling our koi stocking article cites from Texas A&M for a filtered, aerated pond.

UV: clarifier dose, sterilizer dose, and flow

Everything-Ponds, a retailer, gives the rule most retailers repeat: "use 10W of UV light per 1000 gallons for 'clarification', and 30 watts of UV light per 1000 gallons for 'sterilization'." Play It Koi gives the same pair (snippet). A clarifier dose kills the single-celled algae that turn water green. A sterilizer dose also kills bacteria and free-swimming parasites, which take more UV energy, so the watts triple.

Watts are only half of the dose. What an organism absorbs is the lamp's intensity multiplied by the time it spends in the chamber, and that time shrinks as the flow rises. Play It Koi's Aqua Ultraviolet listing (snippet) sizes each unit by the flow "that delivers 30,000 μW/cm² for clarification" or "90,000 μW/cm² for sterilization." Bulk Reef Supply lists the 25 watt Classic for freshwater clarification up to 4,000 gallons and freshwater sterilization up to 1,200 gallons, with flow of "400 (min), 1200 GPH (max)." Everything-Ponds lists the 120 watt Classic as a clarifier for "12,000 Gallons" at "6500 GPH" and as a sterilizer for "4400 - 6000 Gallons" at "4080 GPH." The same lamp sterilizes only when the water passes slowly enough, and then it covers a third to a half of the pond it could clarify.

Two sources scale the UV to fish load directly. Conservation Technology, a pond supplier, rates the 57 watt Aqua Ultraviolet unit at "3200 gph" and matches it to a 5,000 gallon pond at light stock, a 2,500 gallon pond at normal stock and a 1,200 gallon pond at heavy stock. OASE rates its Bitron C 55 W "Suitable for ponds up to max. 60.0 m³", "with fish stock 30.0 m³" and "with koi 15.0 m³" (OASE data on the Cotswold Koi listing), which is 15,850, 7,925 and 3,963 gallons. It sends 6,500 liters an hour (1,717 GPH) past the lamp, and a built-in bypass carries the rest, up to 30,000 liters an hour.

Air

Play It Koi's air pump guide (snippets) calls "1 CFM (cubic foot per minute) of air per 1,000 gallons" the "minimum for koi ponds", which is 28.3 liters a minute. It gives "approximately 40 LPM" per 1,000 gallons for moderate loads and "60 to 80 LPM per 1,000 gallons" for heavy koi stocking, and says to "plan for one diffuser per 2,000 gallons of pond volume." Webb's Water Gardens lists Matala's Hakko HK40LP at a maximum of 70 liters a minute for a pond of "2,000 - 4,000 gal", and Hakko's own page lists its "Rated Performance lpm (cfm): 51," which works out to 13 to 26 liters a minute per 1,000 gallons at the rated output.

All three sized for four ponds

Light stocking means less than 1 inch of grown fish per 10 gallons, which is where Play It Koi draws the line for turning the pond over once an hour. Heavy means show koi or heavy feeding. Each UV wattage is matched to an Aqua Ultraviolet unit with a published flow rating, and the bypass is the filter flow minus what that unit should see.

Filter flow, UV and air for koi ponds from gallons and stocking
Pond	Filter flow	UV watts by rule	UV unit and most flow it should see	Bypass around the UV	Air	Diffusers
1,000 gal, light	1,000 GPH (AD1000 fits, 30 lb)	10 W	15 W, 700 GPH (Conservation Technology)	300 GPH	28 L/min	1
1,000 gal, heavy	2,000 GPH (AD2000 for up to 50 lb)	30 W	25 W, sterilizes 1,200 gal at 400 to 1,200 GPH (Bulk Reef Supply)	800 GPH or more	60 to 80 L/min	1
3,000 gal, light	3,000 GPH (AD4000, 85 lb)	30 W	57 W, 3,200 GPH (Conservation Technology)	none	85 L/min	2
3,000 gal, heavy	6,000 GPH (AD8000 at its 6,000 GPH limit, 165 lb)	90 W	120 W sterilizer, 4,080 GPH (Everything-Ponds)	1,920 GPH	180 to 240 L/min	2
5,000 gal, light	5,000 GPH (AD8000, 165 lb)	50 W	57 W, 3,200 GPH	1,800 GPH	142 L/min	3
5,000 gal, heavy	10,000 GPH (above any single Aquadyne; two vessels)	150 W	120 W sterilizer rated to 6,000 gal, 4,080 GPH	5,920 GPH	300 to 400 L/min	3
10,000 gal, light	10,000 GPH (two vessels)	100 W	120 W clarifier, 6,500 GPH (Everything-Ponds)	3,500 GPH	283 L/min	5
10,000 gal, heavy	20,000 GPH (several vessels or a drum filter)	300 W	two 120 W sterilizers, 8,160 GPH	11,840 GPH	600 to 800 L/min	5

The table shows where the rules collide. At heavy stocking, twice-an-hour turnover outruns a single bead filter by 5,000 gallons, and from there the bypass around the UV carries more than half the flow. The pump that drives the filter flow is its own question, covered in GPH for a koi pond versus an ecosystem pond.

What koi keepers and dealers do

John Fornaro of Hanover Koi Farms, in Properly Sizing a UV Light, sizes by flow: "Do not size a UV based on pond size alone," and on a bypass, send "1/3 of the pond volume" an hour through the unit. For a 1,000 gallon pond that is 333 GPH, just under the 400 GPH minimum flow Bulk Reef Supply lists for the 25 watt Classic. On Garden Pond Forum, koiguy1969 put the mechanism plainly: "flowing thru too fast, just cuts the exposure time."

On filters, keepers oversize. CometKeith on Garden Pond Forum wrote, "don't try to match the size of your pond to the rating of the bead filter. At the minimum you should go at least double or more," and "My pond is about 2500 gallons and I bought one rated for 4000 gallons." Simi Koi says to "cut the number of gallons the filter says it can filter in half."

On air, practice runs from modest to heavy. On Garden Pond Forum, barryian runs a "Hakko40 running 24/7 - 365 days a year with 2 lines" on a 2,000 gallon pond and was "on year 6." On Monster Fish Keepers, Violence, with 13 koi in 4,500 to 5,000 gallons, had been told both 100 and 200 liters a minute, and Muske answered "shoot for the 200 with your fish load," which is 40 to 44 liters a minute per 1,000 gallons.

Where the sources disagree

The UV dose for a koi pond varies by more than a factor of three. For a heavily stocked 3,000 gallon pond, the 30 watt rule asks for 90 watts. OASE's koi rating would cover it with the 55 watt Bitron C, about 14 watts per 1,000 gallons. Conservation Technology's heavy-stock rating needs more than two 57 watt lamps, or 47.5 watts per 1,000 gallons. None of the three publishes the test behind its rating, and each assumes a different flow through the lamp: 1,717 GPH for the OASE, 3,200 GPH for the Conservation Technology rating.

Two numbers circulate without a basis. KoiQuanta's aeration guide says "allow 1-2 liters of air per minute per 1000 gallons," about a twentieth of Play It Koi's minimum, and cites no source for the figure. Pond Informer gives "8w-11w, per 3,000 gallons" to clear green water, about a third of the 10 watt rule. Play It Koi's own heavy-stocking air figure differs between snippets (1.5 to 2 CFM per 1,000 gallons in one, 60 to 80 liters a minute in another), so treat the heavy air column as a ceiling.

Filter gallon ratings carry an unstated fish load, which is why Simi Koi halves them and CometKeith buys a filter rated for double the pond. Aquadyne's pound rating states the load, so check the adult weight of the fish against it.

What to do

Get the volume, then pick the light or heavy row from the fish you will have in five years. More than 1 inch of grown fish per 10 gallons is heavy.
Size the filter by flow and by fish weight, and take the larger. At 5,000 gallons or more with heavy stocking, plan on two filter vessels or a drum filter.
Choose the UV by the flow it will see. Put a valved bypass on the line and set the UV branch to the maker's sterilizing flow, or to a third of the pond volume an hour.
Treat the light-stocking air figure as the least the air pump should deliver, and add one diffuser for every 2,000 gallons, starting at the deepest point.
Test ammonia, nitrite and clarity through the first summer. If the water stays green, slow the UV branch before buying more watts.

The two PondTools calculators that give the inputs

For volume, the PondTools volume calculator (also standalone) takes length, width and average depth in feet and a shape (rectangular, circular or oval, or irregular) and returns gallons. For stocking, the PondTools stocking calculator (also standalone) takes pond volume, surface area, and for each fish type the count and the length now. It grows the fish to adult length and tells you which allowance runs out first. Those two results are the inputs to the table.