Water Gardens – How to Overwinter your Fish

Koi fish on the surface of a pond
Koi fish on the surface of a pond

Water Gardens – Overwintering your Fish

First determine if your pond is likely to freeze solid. If it is less than 3’ deep, most likely it will freeze.

The pond’s location and size of fish and the number of fish all have an effect on the success of overwintering goldfish. Reduce feeding as the temperature cools down in the fall. This usually happens when temperatures fall below 12°C.

Fish metabolism slows down considerably and they may not be able to digest food. The following methods are suggestions for smaller ponds that allow for drainage and capture of fish. When drainage is not practical, as in large of natural ponds, wintering is best left to Mother Nature.


1. Minimize the leaves that fall into your pond. Drain the pond in late fall after the leaves from nearby trees have fallen. Move the fish to covered containers filled with pond water. Clean leaves and other debris from the pond, then refill. Let fresh water sit at least 24 hours to allow chemicals and dissolved gases to dissipate. Place the fish back in the pond. Make sure fish are not stressed during this procedure by keeping their water cool and oxygenated.

2. Remove the fish from the pond and bring indoors to an aquarium or tank for the winter. Keep the tank relatively cool and feed moderately. Oxygenation and filtration may be needed. Return the fish to the pond the following spring, being careful not to shock the fish with a sudden temperature change.

3. Use a pond heater or bubbler during the winter to keep a small area of your pond free from ice. This allows toxic gases that collect under the ice to escape and oxygen to dissolve in the water. Keep in mind that you do not need to run your heater constantly, just enough to keep a small area ice-free.

Be careful that the heater does not touch or melt the pond liner. More fish die from toxic gases trapped in the water produced by vegetation rot than die from freezing. Healthy goldfish and koi have a greater chance of surviving the winter.

Try to minimize stress when overwintering. When transferring fish, ensure water temperatures are similar by floating containers before release. These suggested overwintering methods for fish are frequently used by water gardeners in Canada.

These suggestions can be used alone or in combination with one another to best suit your situation. There is no guarantee that your fish will survive the winter. By trying these commonly used methods your chances for success will be increased.

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