Most koi keepers would agree that properly winterizing your pond before the chilly temperatures hit is the best way to keep your fish safe from the elements, but winter can be unpredictable. Sometimes, even when preventative measures are taken, sudden spells of extreme cold can cause a pond to completely freeze over.

Ignoring an ice-covered pond for too long can be detrimental to your koi. If you wake up one frigid morning to find your pond iced over, all hope isn’t lost. Knowing how to properly de-ice your pond can help you keep your koi safe throughout winter.

When Ice Becomes A Problem

There are a number of reasons your pond may freeze over. Maybe you weren’t expecting winter temperatures to drop so low, and therefore didn’t install the proper equipment. If you are using equipment like an aerator or de-icer to prevent freezing, it may be faulty, incorrectly installed, or not suited for the size of your pond. In some cases, a winter storm could knock out power and cause your equipment to stop working. Whatever the reason, the result is the same: an icy pond.

Koi cannot survive in solid ice. The good news is, it’s rare for a pond to freeze completely solid. Ponds freeze from the top to bottom, so as the surface of the pond freezes, the layer of ice that forms on top of the pond acts as insulation that usually prevents the lower levels of water from freezing.

A thin layer of ice on the surface of a pond can actually be beneficial for koi. It helps keep the water at the bottom of the pond a bit warmer, making it the perfect spot for koi to hangout during the winter months. However, it’s important to keep a hole in the ice to allow oxygen to circulate and prevent toxic gases from building up. Even when preventative steps are taken, extended periods of bitterly cold temperatures can sometimes cause ponds to completely freeze over.

If your pond freezes over for a few days, your koi should be fine. Although the ice will slow down your pond’s oxygen intake and ammonia release, it will not stop the cycle completely. Frozen ponds become a problem if and when they stay ice-covered for an extended period.

If your pond stays ice-capped for longer than a few days, it could harm your koi. Oxygen levels will begin to fall and ammonia and other harmful waste will start to build up, which can be extremely dangerous for your fish.

What To Do If Your Pond Freezes Over

If your pond begins to freeze over and stays frozen for several days, it’s time to take action. Though breaking the ice may be your first instinct, it’s actually a bad route to take. The vibration of the shock waves can harm your koi. The first thing you should do is remove any snow that is lying on top of the ice. It’s best to use a broom to do this in order to avoid any abrasive vibrations that might travel through the water and harm your fish.

Next, you’ll need to create an opening in the ice. The most effective way to do this is by setting a pot of hot water on top of the ice and allowing it to gradually melt a hole in the ice. You can also place a pond de-icer on top of the ice and let it work its magic. If the temperatures stay frigid, check your pond every few days and make sure it hasn’t frozen over again, and repeat the process if necessary. Another option is to gently pour hot water onto the ice. However, because koi are sensitive to temperature change, this method isn’t always advisable.

If you have an aerator and/or a de-icer in your pond, this should be enough to keep the surface of your pond from freezing over. However, if you’re using this equipment and your pond still freezes over, make sure they are working properly and there is no snow covering where the opening should be.

Moving Forward

The cold weather isn’t gone quite yet, and there will likely be a few more bouts of extreme cold before the spring warmth. As we round out winter, make sure your pond is properly prepared for the possibility of more frigid temperatures.


If you don’t have an aerator installed, this is a good place to start. Aerators circulate water, which helps provide oxygen to the pond and prevents the water from freezing by continuously moving the water. If you have an aerator installed and your pond is still freezing over, check the specifications and make sure the model you are using is capable of circulating the volume of water in your pond. If it isn’t compatible, it’s probably a good idea to upgrade to something that is better suited for the size of your pond.


In certain climates, an aerator may not be enough to keep your pond from freezing over. If your aerator alone isn’t keeping a hole in the ice, consider pairing it with a quality de-icer. Most de-icers are controlled by a thermometer and will automatically turn on when water temperatures get close to freezing, which can help put your mind at ease in the event of a sudden temperature drop. Keep in mind that de-icers are not heaters. Though they do produce heat, it’s only enough to prevent the pond from freezing over completely. There are two main types of de-icers: floating and submersible. The type that is best for you largely depends on the depth of your pond.

Submersible de-icers can help heat a small portion of your pond. The deeper your pond, the warmer the bottom of it will be. Since ponds freeze from top to bottom, shallow ponds freeze more quickly. If your pond is 3” or shallower, a submersible de-icer will help keep the bottom of your pond a bit warmer for your koi and prevent them from freezing; however, there is no guarantee that a submersible de-icer will keep an opening on the surface of your pond. In this case, you may also need to purchase a floating de-icer. Floating de-icers sit on the surface of the water to ensure a hole is kept open in the event of ice formation. If you have a deeper pond and aren’t worried about the bottom of your pond getting chilly, a floating de-icer is probably the best choice for you.

Just like with an aerator, when purchasing a de-icer make sure the unit is well-suited for the size of your pond. While size recommendations vary depending on the manufacturer of the product, typically larger ponds require greater wattage.

Other Tricks

In addition to ice-prevention equipment, there are also a few do-it-yourself tricks that can assist in ice prevention. One option is to place a few tennis balls or a beach ball in water. The motion of the balls will help keep the water moving and prevent it from completely freezing. This can also be done with an empty two-liter soda bottle or a pool noodle. Also, installing protective netting over your pond can help keep the surface of your pond free from leaves and other debris, which will help limit the build-up of waste in the event your pond does become ice-capped.

The most effective way to keep your pond ice-free is by taking preventative measures before the freezing temperatures hit. What tricks have you developed for keeping your koi cozy during the winter? Let us know in the comments!

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