In Northern states where the water in a pond will freeze over, special care needs to be taken to help fish such as koi, comets, fantails and others to survive. These fish are all hardy for most northern areas but conditions in a small backyard pond put extra stresses on them. 

Fish are cold blooded creatures. When the ambient temperature of the pond water drops below a certain minimum, they become inactive and drop to the bottom of the pond. Once the water is below 50 degrees F, stop feeding the fish even if they continue to come to the surface and beg. At lower temperatures, the digestive system of the fish does not work fully and partially digested food may cause a blockage. This could result in death of the fish.

The key reason that fish die in the winter is that the "gunk" on the bottom of the pond continues to decompose slowly in the cold water. In the process, it releases gases such as ammonia and others. If the surface of the water is not frozen, this gas simply escapes into the atmosphere. If the pond surface is frozen solid, the gas is trapped and may build to lethal levels resulting in the death of the fish.

Most people either try to keep the fish in the pond but a few take the extra step of creating a pond indoors and bringing the fish inside for the winter. 

In the Pond 

First, to keep the fish in the pond, it needs to be at least 18 inches deep. Two to three feet deep provides and extra buffer. If you live in an area where natural lakes and ponds form a foot or more of ice each winter, you really need to use a deeper pond if you want to keep them outside.

If the pond depth is adequate to prevent freezing solid, the next concern is the build up of toxic gases. Fish droppings and plant materials such as leaves will produce toxic gases such as ammonia when they decompose. 

When the surface of the pond is open, these gases merely disperse into the air. But, if the pond is covered with ice, they are trapped and may build to toxic levels. 

To prevent such buildup, many people use an air bubbler to keep part of the surface of the pond free of ice. In severe weather, this may need to be supplemented with a stock tank heater. Avoid using the tank heater all winter unless you have stock in the local utility company.

Keeping the pond clean and well filtered will reduce the potential amount of toxic gases produced. If the pond is clean going into the winter and you live in an area where the pond is only frozen for short periods, the fish may survive without any extra help.

Remember that fish are cold-blooded creatures. They get their body heat from the temperature of the water. Their digestive systems do not begin to fully function until the water temperature is above 50 degrees F. Although they may be swimming around the pond in colder temperatures, avoid the temptation to feed them until the pond reaches 50 degrees. If their body temperature is too cold when they are fed, you run the risk of having their digestive tract clogged up resulting in their death.

In the House 

1. If you have especially valuable fish or if winters in your areas are extremely severe, you can always bring your fish indoors. A large livestock watering tank or a structure made of wooden frame with pond liner may be used to hold the fish in the basement.

2. You will need to have some type of water circulation system and some filtration. Depending on the temperature of your basement, you should probably cut back on the amount of feed which will help cut back on other problems.

3. In the spring when your outdoor pond is above 50 degrees, you can reintroduce the fish. Whether you transport them in a plastic container or bag, allow the container to float in the water for several hours to slowly equalize the temperature. This will help to avoid shock to the fish during rapid temperature changes.