Fish like koi and goldfish enter a semi-dormant state called torpor when the water temperature drops for the season. Torpor is not quite hibernation, but your fish's activity level slows drastically once they enter this state.
So what does that mean for you as their caretaker?
At a Glance: Winter Pond Fish Care
- Koi, goldfish and most other pond fish enter a semi-dormant state called torpor in the winter.
- Fish do not need to be - and should not be - fed in cold temperatures.
- Aeration is key for ensuring your fish make it to spring.
- Most pond fish will survive cold winters given the proper care.
Should I take my fish inside for the winter?
Spending all winter stuck under ice doesn't sound fun to us, but that doesn't mean you have to bring your fish inside to warm up by the fireplace.
Most fish - including koi, goldfish and catfish - can weather the cold just fine thanks to their amazing biological safeguards.
Think about it this way: In the wild, carp (an ancestor of koi and goldfish) and lots of other creatures make it through the winter without any help from humans. Your domesticated fish manage just as well.
Do I need to feed my fish in the winter?
Pond fish do not need food when water temperatures are consistently below 55 degrees. Don't worry; they won't go hungry when you pack up their pellets for the season.
During torpor, fish's metabolisms slow as their bodies conserve as much energy as they can for the winter months. Most of the food they eat during this semi-dormant state will sit undigested in their stomachs, meaning you risk making your fish sick if you feed them after the mercury drops. Even if your fish don't eat food offered to them, any pellets you throw in the pond could sink to the bottom and create water-quality issues.
You can make the transition into and out of torpor easier for your fish by using a specially formulated cold water food in the fall and spring. This food, available at Splash, has less protein and more wheat germ than summer formulas. Your fish can easily digest it as their metabolisms wind down for the season.
Switch to cold water food when temperatures start to feel consistently cool but your water is still usually above 55 degrees. Keep an eye on how much your fish consume in about a five-minute period, and gradually cut back the number and size of feedings as they seem less interested in eating. Gradually bring the food back as your pond warms up in the spring.
How do I aerate my winter pond?
Your semi-dormant fish aren't eating, but they're still exhaling. Disturbing the water surface with a waterfall or a pond aerator as the temperatures drop below freezing is critical for fish health so those gases don’t get stuck under ice and suffocate your finned friends.
Many ponds can benefit from additional aeration in the winter in the form of an aerator. In extreme cold, you might want to also consider using a pond de-icer: a heater-like device that keeps your pond from freezing over while still keeping the water at an appropriate winter temperature.
What if your pond is already frozen-over? In that case, you'll need to make a hole in the ice yourself - but be gentle. Smacking the ice with a blunt object, like a shovel, will send vibrations through your pond water that could damage your fish's lateral line (the organ they use to sense things like depth and orientation when swimming). Try thawing the ice with a de-icer or pot of boiling water instead, or carefully cut through it with a rough-toothed saw.
My koi are sitting at the bottom of the pond. Are they dead?
Imagine you wander into a bear's den in the winter and see it laying motionless in the corner. Would you poke it to see if it were dead? Probably not. You know the bear is just hibernating.
Your semi-dormant pond fish look a lot like that bear: not moving or doing much of anything, but still very much alive.
During torpor, koi often hang out at the bottom of the pond and move very little. In smaller ponds, you might even notice them all huddled up in one small spot - usually the warmest pocket of water they can find. They'll continue this lack of activity throughout most of the winter, maybe waking up for a slow swim on warmer days. Don't worry; they're not dead. They'll be back to their regular fishy activities as soon as the warmer days settle in to stay.
Speaking of warmer days - don't forget that early spring is the best time to clean your pond if it needs it. If you wait until later in the season, you risk wiping out the beneficial bacteria colonies that start to re-establish themselves in warmer weather. Click here to learn more about Spring Pond Clean-Outs.