Winter pond care2026-10-04

How to winterize a koi pond: pump on or off, de-icer or aerator

In a cold climate, most owners shut the waterfall pump off for winter, store it in a bucket of water indoors, and keep a hole in the ice with a small aerator 1 to 1½ feet below the surface, adding a thermostatic de-icer where winters are hard. Feeding stops somewhere between 50°F and 40°F water, depending on whose chart you follow. Hardy plants are cut back and sunk to the bottom, and the leaf net comes off once the leaves are down. The fish need water that never freezes solid, which the sources put at 18 to 24 inches in the Deep South and 3 to 5 feet in Minnesota.

What the manufacturers and extension sources say

On the pump, The Pond Guy and Aquascape disagree. The Pond Guy's Winter Pond Pump Use says "We recommend storing your pond pumps in winter to prevent the risk of damage to your pump and water garden," and warns that "Ice dams can develop and divert water from the pond." It says to store the pump "in a five-gallon bucket filled with water to keep the seals lubricated." Aquascape's pages refuse automated fetches, so its position comes from search snippets of its winter care pages: "A pump that provides at least 2,000 gph can be operated throughout the winter without a problem, as long as it runs continuously," though "repeated days in sub-zero temperatures may lead to excessive ice build-up."

On aeration, Aquascape's own manual for its Pond Air kits, reproduced by Splash Supply, is the most specific source we found. It says not to put the discs on the bottom in winter, because that "may disrupt the natural thermocline" where the fish overwinter, and to "place the Aeration Discs about 1 to 1 ½ feet below the surface of the water." The same manual says the discs "can be placed beneath the de-icer." The Pond Guy's winter aeration tips say to put diffusers "in the shallows or near the surface." The Minnesota Water Garden Society's winter care page puts the diffuser "at the middle of the depth range (not the deepest part)" and says a hole "About 12 inches in diameter is usually sufficient."

On feeding and plants, the Southern Regional Aquaculture Center's Water Gardens fact sheet (SRAC 435) says "Fish do not feed at water temperatures below 45°F." For plants, it says tropicals "won't survive winter freezes and must be brought indoors," and that hardy lilies and iris "are winterized by cutting off old growth and placing their pots in the bottom of the water garden (below the freeze line)." Aquascape, in a snippet of its fall feeding page, says to stop feeding at 50°F or lower. The Pond Guy's fall shutdown guide says that "When water temperatures fall below 50°F, reduce feedings to three times per week," and to stop below 40°F.

On netting, Aquascape (snippet, Keep Your Pond Free of Leaves) says to net before the leaves fall and, "Once all the leaves have fallen, simply roll up the net."

De-icer wattage by climate and pond size

K&H is the only manufacturer we found that sizes its de-icers by climate. Its zone chart is an image on its product page, and Pond Warehouse lists it as text:

K&H Thermo-Pond de-icer wattage by USDA zone and pond gallons (manufacturer chart, as listed by Pond Warehouse)
Zone	0 to 100 gal	100 to 400 gal	400 to 750 gal	750 to 1,000 gal	1,000 to 1,200 gal
Zone 3	250 W	750 W	750 W	1,500 W	1,500 W
Zone 4	250 W	750 W	1,500 W	1,500 W	1,500 W
Zone 5	250 W	250 W	750 W	750 W	1,500 W
Zone 6	250 W	250 W	250 W	750 W	750 W
Zone 7 and up	250 W	250 W	250 W	250 W	750 W

The other ratings we found are by gallons alone, and none of them is by surface area:

Other published de-icer ratings
Source	Rating
The Pond Guy sizing guide	Up to 250 gallons, 100 to 250 W; 250 to 500 gallons, 300 W; 500 to 1,000 gallons, 750 W; 1,000 to 1,500 gallons, 1,250 W; 1,500 to 2,000 gallons, 1,500 W
Laguna PowerHeat 315 W	Rated to 1,500 gallons (AZPonds listing)
Aquascape 300 W	No gallon rating; "In extreme conditions or gardening zones above USDA zone 5 we recommend the deicer be used in conjunction with an Aquascape pond aerator" (123Ponds listing)

Every chart stops at 2,000 gallons or less. For a 3,000 to 10,000 gallon pond, no manufacturer we found gives a number. Minnesota Waterscapes describes the job this way: "A pond de-icer won't heat the water; it will simply keep a small hole open in the ice."

Running cost is easy to work out. At the July 2026 U.S. average residential price of 18.31 cents per kilowatt hour (EIA Table 5.6.A), a 300 W de-icer running every hour of a 30 day month uses 216 kilowatt hours, about $39.55. A 1,500 W unit uses 1,080 kilowatt hours, about $197.75. The thermostat cycles the heater, so those are ceilings. Aquascape's 11 W four-outlet aerator, which the manual rates for "very small backyard ponds up to 800 gallons," uses 7.92 kilowatt hours, about $1.45.

What contractors and owners do

In the Twin Cities, Minnesota Waterscapes reports that "Roughly 70 percent of pond owners in the Twin Cities area of Minnesota, decide to shut down their system." Owners who keep the stream running should expect "to go out a few times a month during the winter to 'top off' the pond." Premier Ponds of Glenwood, Maryland, writes that "Small ponds under 500 gallons can usually get by with a 250 or 300-watt unit. Larger ponds up to 1,500 gallons may need 1,250 watts or more," and gives its own area rule: "Use one pond de-icer per 150 square feet of surface area."

Owners on Garden Pond Forum mostly go small. In a 2014 thread, JBtheExplorer in southeastern Wisconsin found his 1,250 W unit had held an 800 gallon pond at 42°F on an 11°F night ("there was no ice on the pond"), while Tula in Cincinnati said of a 120 W Pond Master, "it just keep a hole open in the ice." In a 2022 thread, mrsclem in Maryland wrote "The 1500w heater is going to kill your electric bill!" and now uses pond breathers and 100 W thermostatic heaters. In a 2016 thread, addy1 in Frederick, Maryland, said "We have frozen down 18 inches, but that is unusual for us."

The one practitioner who dissents on placement is Jamie Beyer of Midwest Waterscapes in Boone, Iowa, answering in Pond Trade Magazine in 2010. He says to "place that aeration point on the bottom of your pond, in the center," and writes that "the water will get to 32.1°F on the bottom of a 5.5´ pond in a typical aerated Midwestern Koi pond." He agrees on the risk: "If you start heavy aeration after the ice is already formed then you can possibly super cool a pond."

Where the sources differ and why it matters

Depth is the clearest conflict. The Pond Guy says "at least 18″ to overwinter hardy fish," and its own overwintering page says that "water can easily freeze 18″ to 24″ deep depending on weather conditions." Both can be true only in a mild winter.

Overwintering depth by region, with the source for each row
Region	Depth given	Source
Deep South	18 to 24 inches, "as long as only a few fish are stocked"	SRAC 435
General minimum	At least 18 inches	The Pond Guy; K&H 750 W de-icer listing (AZPonds)
Maryland	About 2 feet; 18 inches of ice is "unusual"	Premier Ponds; addy1, Garden Pond Forum
Year-round ponds in cold climates	A section "at least 3 or 4 feet deep that does not freeze"	SRAC 435
Minnesota	3 to 5 feet or more	Minnesota Water Garden Society

Our article on how deep a koi pond should be covers the build side of this.

On placement, Aquascape's fish care page (snippet) says to keep the pump and the de-icer apart, or the de-icer "will run continuously" heating circulated water. Its aerator manual says the discs can go beneath the de-icer. Both agree the air belongs well above the bottom, and only Beyer disagrees. For acre ponds, where the diffuser question is about winterkill under snow, see our winterkill article.

One sizing rule circulates without a manufacturer behind it: 1,000 W per 100 square feet, published by calculator sites such as PondCalc with no source cited. Pond Trade Magazine's heaters versus aeration article by Harrison Hugo notes that "The operating cost of a 1250-watt heater can easily run over $100 a month," against "$2 to $3 per month" for aeration.

What to do

Net the pond before the leaves drop, and take the net off once they are down. Pull out what got through, since Aquascape says decaying leaves "produce toxic gases that can harm your fish."
Check the pond's coldest-water depth against the table above. If it is shallower than your region needs, bring the fish in or plan on a de-icer and aerator from the first hard freeze.
Cut feeding back below 50°F water and stop by 45°F at the latest, which is where SRAC says the fish stop eating.
Cut hardy plants back and sink their pots to the bottom. Bring tropicals indoors.
Run the stream only if the pump moves at least 2,000 gph and runs without stopping, and someone will watch for ice dams and top the pond off. Otherwise pull the pump, clean the skimmer and filter, and store the pump in water where it will not freeze.
Start the aerator before ice forms, with the stones 1 to 1½ feet down on a shelf, never on the bottom.
Add a thermostatic de-icer from the charts above. Past 2,000 gallons, the largest chart figure for your zone is a ceiling, because the de-icer only has to keep a hole open.

Do not start heavy bottom aeration after the ice is in, and judge feeding by water temperature, measured with a thermometer, rather than the air.

The PondTools calculators for this

Most de-icer charts ask for gallons, so start with the PondTools volume calculator (also standalone), which takes length, width, average depth and a shape setting and returns gallons. Then the PondTools surface-area calculator (also standalone) takes the surface area in square feet, the typical water temperature and the coldest night you plan for, and returns the air flow in CFM and a de-icer size. Its de-icer figure is the surface area times 7, 10 or 15 watts per square foot for mild, cold or severe winters, and at the cold setting that is exactly the 1,000 W per 100 square feet rule above. A pond of 200 square feet, about 3,000 gallons at 2 feet deep, comes out at 2,000 W in a cold winter, which is above every manufacturer chart. Read it as the upper bound and buy to the manufacturer's rating. If you trace the pond in the PondTools app, the gallons and square feet come straight from the outline.