While there isn't much going on externally in winter time pond-keeping, it is important to take certain steps to ensure the health of the pond and fish. With this proper care, you can help make sure the fish not only survive winter, but get a good start going into the following spring season.
Feed fish appropriately. The water temperature is dropping now and we should be feeding our fish less as their metabolism slows down. Hopefully you have been feeding your fish well with a high protein food this summer to allow them to build up a reserve of fat to help them through the winter. After the water temperature drops below 60 you should decrease the amount of food given and feed only once a day. A wheat germ based food is recommended as it is easily digested. Microbe Lift Cold Weather is an excellent food at this time of year. Or if you want a sinking pellet then the Hikari Sinking Wheat Germ would be the way to go. As the water temperatures continue to drop to below 55 degrees you should feed only two or three times a week. It can take your fish two or three days to digest food at this temperature. Once the temperature drops below 50 degrees you should stop feeding altogether until spring when the water temperature remains above 50.
Ensure proper gas exchange. As organics decompose in the pond they can produce toxic gases that could be trapped in the pond if it is covered by ice. It is important to keep at least a small area free of ice so that these gases can escape. This also allows oxygen in. Do not break the ice as the shock waves created can damage or kill your fish.
In warmer climates (approximately USDA zones 7 and up) many people simply leave the waterfall running. This will keep the surface from completely freezing over.
In moderate climates (approximately USDA zones 5-6) the waterfall will be turned off, but we can use an aerator to keep a hole in the ice. You hopefully already use an aeration pump in the summer, simply place the diffuser into shallow water. This option can also be used in the warmer areas if desired.
In colder climates (approximately USDA zone 4) this is with a floating pond de-icer. This device floats in the pond and has a built in thermostat to turn the heating element on when the water temperature drops below 40 degrees. They can also be used to keep a small pond from freezing solid allowing you to keep your fish alive. This option can also be used in warmer areas. But we don’t want the waterfall running in this climate as it can super-chill the water.
Areas colder than zone 4 may wish to consider a complete shut-down of the pond and bringing fish indoors.
Maintain a healthy environment. A clean and healthy pond goes a long way to reduce the stressful effects of winter. A big part of this is trying to minimize the sludge and debris in the pond. Covering the pond with leaf netting in fall is one of the easiest and most important steps. Also, transitioning to a beneficial bacteria that is effective in colder temperatures helps ensure the ecosystem stays at least somewhat active.
Dying foliage from your pond plants should be cut off and removed as best you can. The less debris allowed to settle to the pond bottom, the healthier the pond can be.
Protect your fish. With their slower metabolism and the absence of plants our fish are more susceptible to predation by raccoons, birds and other animals. If you took our advice and added leaf netting to keep the leaves out of your pond this should protect your fish as well, it is generally best just to leave the netting down all winter. A Koi Kastle will help your fish be more comfortable by providing a place for them to hide.