How to Overwinter a Pond and Its Fish

The short version

• Keep one section of the pond at least 2 ft deep. That depth holds liquid water below the ice where fish overwinter.
• Stop feeding once the water settles below about 50 F. In cold water a goldfish barely digests, and uneaten food fouls the pond.
• Keep a hole open in the ice all winter so gases escape. A fully sealed surface, not the cold, is what kills fish.
• Hardy plants like a water lily (50 to 86 F) stay in the pond below the freeze line; tender tropicals come out or die.
• Cut and net out dying foliage in autumn so it does not rot under the ice and burn through the oxygen.

A pond and the fish in it survive winter in place, as long as one spot stays at least 2 ft deep and one hole stays open in the ice. The cold itself is rarely the killer. What kills fish is a surface that freezes over solid and seals the pond, trapping the gases that leaf rot and fish respiration release until the water underneath turns toxic.

So overwintering is less about heat and more about two things: depth, which gives the fish liquid water to rest in, and an open hole, which lets the pond keep breathing through a freeze. Get those right and a goldfish that spent summer at the surface will sit near the bottom for months and rise again in spring. Here is the full winter setup, and the ways it goes wrong.

The real winter threat is the ice, not the cold

Cool-water fish are built for winter. Goldfish (Carassius auratus) are active at 60 to 74 F and simply slow down as the pond cools, dropping to the bottom and resting through the cold with almost no food. White cloud mountain minnows (Tanichthys albonubes), happy at 60 to 72 F, overwinter the same way in a pond deep enough not to freeze solid.

The danger is what happens above them. When ice caps the entire surface, the carbon dioxide from the fish and the gases from decaying leaves have nowhere to go, and oxygen cannot get in. Over weeks under a sealed lid, that buildup can suffocate the fish even though the water never froze around them. Every winter step below exists to prevent that one outcome.

Depth: the 2 ft rule that does the work

Depth is the cheapest insurance a pond has. A section at least 2 ft deep holds a pocket of liquid water that stays near 39 F at the bottom even when the top is frozen, because water is densest just above freezing and sinks. That deep pocket is where the fish spend winter.

A preformed shell, a lined dug pond, or a large stock tank all work, as long as one part of the shape hits that 2 ft low point. A shallow dish under a foot deep can freeze most of the way through in a hard winter and leaves the fish nowhere to go, which is why a shallow container pond in a cold climate is emptied and the fish moved indoors instead. If your pond is not 2 ft deep somewhere, that is the first thing to fix before you count on it overwintering anything.

The fish: stop feeding and let them rest

The single most important winter task is to stop feeding. As the water falls below the fish's active band, digestion slows, and many keepers stop feeding entirely once the pond holds below about 50 F, because a goldfish's gut nearly halts in cold water. Food fed past that point sits undigested, fouls the fish, and rots in the pond.

Taper feeding through autumn as the water cools, then leave the fish alone. They are not starving; they are dormant, living off stored reserves at a near-stopped metabolism until spring warms the water back into their range. A pond stocked within its limits (remember a goldfish carries a high bioload and wants a practical 30 gallons each) has enough oxygen and space to carry its fish through a still winter. A fish that looks unwell heading into the cold is a question for a veterinarian who works with fish, not a gamble on it pulling through the freeze.

The plants: hardy stays, tender comes out

Pond plants split cleanly into two winter groups. Hardy plants overwinter in the pond: a hardy water lily (Nymphaea odorata, 50 to 86 F) is cut back and its pot lowered to the deep zone, where it sits below the freeze line and returns in spring. Native marginals do the same, dying back to roots that survive on the shelf if the crown sits below the ice.

Tender tropicals do not survive a freeze and come out. Water hyacinth (Pontederia crassipes), which wants 65 to 86 F, is killed by the first hard frost and is treated as an annual in cold climates, composted rather than saved. Parrot's feather (Myriophyllum aquaticum, 50 to 85 F) is hardier and can persist in a deep enough pond, but in a hard winter it too dies back. Whatever you keep, cut the dying foliage before it collapses into the water.

Keeping a hole in the ice

If your winters freeze the surface, you keep one area open. A floating de-icer (a thermostat-controlled float that keeps a small patch, roughly 6 to 12 inches across, thawed) is the common tool, and a small air stone run near the surface also holds a hole open by keeping the water moving. The goal is a gap of open water for gas exchange, not a warm pond.

Never smash the ice to open it. The shock wave travels through the water and can injure or kill dormant fish resting below. If the surface has already sealed over, set a pot of hot water on the ice to melt a hole gently, or run a de-icer from then on. One open patch is all the pond needs to keep venting gases through the coldest stretch.

A fall checklist, in order

Run these in sequence as the season turns, roughly when the water drops through the 60s and into the 50s F:

1. Net fallen leaves off the surface weekly through autumn, and cover a small pond with netting during heavy leaf-fall so debris never reaches the bottom.
2. Cut back and lower hardy plants (the water lily pot goes to the 2 ft zone); pull and compost tender tropicals like water hyacinth.
3. Taper feeding as the water cools and stop entirely once it holds below about 50 F.
4. Set up a de-icer or a surface air stone before the first hard freeze, not after.
5. Test the water with a liquid kit in autumn and again in spring, so you catch an ammonia or nitrite spike early on either side of winter.

The honest part: how a pond kills fish over winter

Nearly every winter loss is one of three failures, and all three are design, not bad luck. The first is a sealed surface: ice caps the whole pond, no hole is kept open, and the fish suffocate under trapped gases over weeks. A de-icer or air stone prevents it for the cost of a few dollars of electricity.

The second is rotting vegetation. Leaves and dying plants left in the pond decay under the ice, consuming oxygen and releasing gases into water that cannot vent, which pushes a sealed pond toxic much faster. Netting leaves out in autumn is the fix. The third is too little depth: a pond under 2 ft freezes too far down and leaves the fish no liquid refuge. None of these is the cold doing the killing; each is a surface, a leaf load, or a depth you can correct before the first freeze.

Frequently asked questions

Do I need to bring my pond fish indoors for winter?

Not if the pond has a section at least 2 ft deep and you keep a hole open in the ice. Cool-water fish like goldfish (active at 60 to 74 F) and white cloud minnows (60 to 72 F) overwinter in place, resting near the bottom. Fish only come indoors when the pond is too shallow to keep liquid water below the ice, as with a small container pond under 2 ft deep.

Should I run the pump and filter through winter?

In a hard-freeze climate, most keepers shut the pump down and pull the filter, because pumping cold surface water down can chill the deep zone where the fish rest. A separate de-icer or a surface air stone keeps the gas-exchange hole open instead. In a mild climate that never freezes solid, you can leave a pump running.

When do I stop feeding pond fish?

Taper off through autumn and stop entirely once the water settles below about 50 F. Below that a goldfish's digestion nearly stops, so any food you offer rots in the gut and in the pond rather than feeding the fish. They live off stored reserves through the cold and start eating again when spring warms the water back into their range.

Will a de-icer heat the whole pond?

No, and it is not meant to. A floating de-icer only keeps a small patch of the surface from freezing so gases can escape; the rest of the pond still ices over and the deep water still sits cold near 39 F. That open hole is the entire job, since the fish do not need warmth, only a way for the pond to keep breathing.

Overwintering is a setup you finish before the first freeze, not a rescue you run during one. Run your pond's depth and stocking through the build planner to check it can carry its fish through a cold season, and pick the oxygenators and hardy plants that overwinter with the oxygenating pond plants guide. If you are starting small, a stock tank pond needs its fish moved indoors in a hard winter, while a deeper self-sustaining pond holds them in place. Check your fish and plants against the compatibility database for their cold-water ranges, size a cool-water stock with the goldfish pond setup, and the rest of the pond guides carry the next season.