Should You Run Your Pond Pump in Winter?

"Should I run my pond pump in winter?" is one of the most-asked and worst-answered questions in the hobby, because the honest answer is "it depends," and most of the advice online picks a side and skips the reasoning. Leave the pump running and you keep your filter alive and a hole open in the ice. Pull it and you protect the still, warm layer of water your fish overwinter in. Both can be right, and both can be fatal, and which one is right for you comes down to your pond's depth and your climate, not to a blanket rule.

Here is the part the "just leave it running for filtration" articles leave out, and the reason this question matters at all. A deep pond in winter separates into layers, and the densest water, at about 39F, sinks to the bottom, where your koi drop into near-dormancy and wait out the cold. That bottom layer is their refuge. Run a waterfall or a pump that pulls from the bottom all winter, and you mix that warm layer up into the freezing surface air, cooling the entire pond toward 32F and stripping your fish of the one place they were safe. That is not a filtration decision. It is the difference between fish that walk into spring and fish that do not. This guide walks the real decision, deep versus shallow, mild versus hard freeze, and tells you what to run instead when the answer is to pull the pump.

First, the thing that keeps fish alive: an open hole

Before the pump question, understand what kills fish under ice, because it is not the cold. Fish are cold-blooded and overwinter fine in near-freezing water. What kills them is a pond that ices over completely and stays sealed for days: the fish and the decomposing debris on the bottom keep giving off carbon dioxide and ammonia, and a solid lid of ice traps those gases in while shutting oxygen out. The water goes toxic and the fish suffocate under a frozen surface that looks perfectly calm from above.

So the real winter job is not "run the pump," it is "keep at least one hole open in the ice for gas exchange" all winter. You do that with a floating de-icer, an air pump feeding a stone set near the surface, or both, and in a mild climate a trickle of pump flow can help hold it. Get that hole handled first. Everything about the pump below is a question of how you keep that hole open without chilling your fish.

The rule, by pond depth and climate

With the open hole understood, the pump decision comes down to two variables. Read yours off this and you have your answer.

• Deep pond (4 to 5 ft) in a mild-to-moderate winter: you can keep the pump running, with two changes. Raise the intake off the bottom so you are circulating only the top of the pond and leaving the warm 39F layer undisturbed, and shut the waterfall off for the season. Gentle surface circulation helps hold a hole open and keeps your filter bacteria fed, and a deep pond has enough warm water at the bottom to shrug off the mild mixing.
• Deep pond in a hard-freeze climate: pull the main pump, or run only a small surface-level pump you can trust not to reach the bottom. When the air sits below freezing for weeks, any real circulation will super-cool the whole column. Rely on a de-icer and a high-set air stone to hold the hole, and let the deep water lie still.
• Shallow pond (under about 3 ft), anywhere it freezes: pull the pump, every time. A shallow pond has no deep warm layer to protect, so it will freeze toward the bottom, and a running pump only speeds that up. Winter is a de-icer-and-air-pump job here, not a pump job, and in a truly hard freeze a shallow pond may not safely overwinter fish at all.
• Any pond, if you will not check it daily: lean toward pulling the pump. A running pond in winter needs eyes on it, a de-icer or a pump can fail in a cold snap, ice can dam a waterfall and pump the pond dry, an intake can freeze. If you are not going to be out there checking, the simpler, lower-risk setup is a pulled pump plus a de-icer.

Why a running waterfall is the classic fish-killer

If you take one thing from this guide, make it this: shut the waterfall off for winter. A waterfall is the most efficient way there is to chill a pond, because it throws the water into the coldest air you have, spreads it thin over rocks, and pours it back in colder every pass. In a hard freeze it will drive the whole pond toward 32F and destroy the warm bottom refuge in a night or two. It also builds ice dams that divert the flow and can silently pump your pond dry through a channel in the ice, leaving the pump running in air and the fish in a puddle.

This is the single most common way well-meaning keepers lose fish over winter: they leave the pretty waterfall running because the pond "has always run," and a cold snap does the rest. Circulation for a hole is fine, in the right pond, at the surface. A waterfall crashing into freezing air is a different thing, and it belongs shut down from first hard frost to spring.

What about the filter bacteria? (the overrated worry)

The most common reason keepers give for running the pump all winter is to keep their biological filter alive, and it is a weaker argument than it sounds. Below about 50F the nitrifying bacteria that run your filter go dormant, not dead; the cold alone does not kill the colony. And your fish, near-dormant themselves, are barely eating and barely producing the ammonia the filter exists to process, so the filter has little to do in deep winter regardless.

What actually kills a filter colony over winter is not shutting the pump off, it is one of two mistakes. Leaving the filter sitting full of stagnant, unaerated water suffocates the bacteria within hours, so if you pull the pump, do not leave the media soaking in a dead filter. And letting the media freeze solid kills it outright. The clean answer if you shut down: drain the filter, and either keep the media somewhere cool, moist, and frost-free, or sink it in the pond near the fish for the winter and lift it back in spring. Either way the colony re-establishes on its own as the water warms in spring, exactly as it did when your pond first cycled. Do not let "saving the bacteria" talk you into churning your pond all winter; the bacteria are the easy part.

If you pull the pump, store it right

A pump that is left to dry out over winter is a pump with cracked seals and a seized impeller come spring. When you pull it, rinse it off and store it submerged in a bucket of water somewhere that will not freeze, a basement, a garage that stays above freezing, a heated shed. Keeping the seals and impeller wet is what lets the pump start cleanly next season. Never let a pulled pump sit dry, and never let the bucket freeze with the pump in it.

The cost angle, briefly

There is also a plain-money reason not to run a pump all winter for no reason. A typical pond pump draws real wattage around the clock, so running one for months to serve a filter that is dormant and fish that are barely producing waste is paying to move water you do not need moved. If your climate lets you pull the pump and hold the hole open with a de-icer and a small air pump, you will spend far less over the winter and take on less risk. Run the pump in winter because your pond benefits from it, not out of habit.

Winter pond questions, answered

Will my pond pump freeze if I leave it running?

A submerged pump running in deep enough water will not freeze, moving water and the warmer deep layer keep it clear. The risk is not the pump icing up, it is the pump running dry: if ice dams a waterfall or the water level drops through a channel in the ice, the pump can end up pumping air, which burns it out and empties the pond. That failure mode is exactly why a running winter pond needs daily eyes on it, and why pulling the pump is the safer default if you cannot check it.

Should I turn off my pond waterfall in winter?

Yes. Shut the waterfall down from the first hard frost. A waterfall is the fastest way to super-cool a pond and destroy the warm bottom layer your fish overwinter in, and it builds ice dams that can pump the pond dry. If you want winter circulation to help hold a hole open, keep it gentle and at the surface, not a waterfall crashing into freezing air.

Do the bacteria in my pond filter die over winter?

No, they go dormant below about 50F, not dead, and re-activate in spring as the water warms. The cold does not kill them. What kills a filter colony is letting the media freeze solid, or leaving it sitting in stagnant, unaerated water after you stop the pump. If you shut down for winter, drain the filter and keep the media cool, moist, and frost-free, or sink it in the pond.

Where should the pump sit in a pond in winter?

If you keep it running, raise the intake up off the bottom so it circulates only the upper water and leaves the warm 39F layer at the bottom undisturbed. Never run a pump that pulls from the very bottom of a deep pond in winter, that is what strips the fish of their warm refuge. In a shallow pond, the better move is to pull the pump entirely.