Don’t break the ice!
Have you ever seen someone tap their finger on the glass of an aquarium? The fish jump, the sound waves scare the crap out of them. This is not healthy for fish, for some reason making loud noises affects fish negatively, causing them to become susceptible to bacterial infections and viruses. Were am I going with this? It is winter and if you have fish in your pond you probably know that a frozen over pond or water garden often has fish-kill, (fish that die during the winter). Some people think they see the fish under the ice which don’t appear to be moving are actually dead, frozen in the pond.
Unlikely– unless the pond is frozen to the bottom. Though your fish can die with a quarter inch of ice, they will not die from the cold. They die from both the lack of oxygen as they use up available oxygen in the water and no new oxygen can penetrate the ice. If your pond freezes over in one night and thaws the next day, it’s nothing to worry about– the fish are probably never going to notice.
Keep a pump running that will provide a hole in the ice. Often a pump upwelling from the ledge will provide a strong current upwards. It will however cool the water to a much lower level at the bottom of the pond. Also make sure your leaves & debris are out of the pond before winter, you don’t want your pump getting clogged when its 10 degrees outside and you having to go unclog it. An aerator that blows air under the water will keep a hole in the ice as the water is always moving where bubble are coming up. These are sometimes more expensive or need attached to an existing pump.
Last of all, I would like to disprove the myth of fish freezing in ice during the winter. Every year I hear this from people, “I can see them in the ice, they aren’t moving, they are frozen and come back to life when it thaws”. This is by no way true or scientists would be studying the first fish that did this. The truth is that as the water gets cold the fishes metabolism slows down to the point where the fish just basically sits in the pond with his gills moving. They are winter couch potatoes, go out every few days they will have changed position a bit and every thaw they will swim around a little. The metabolism is so slow you cannot ever feed fish Nov-April. When I was young ,on the first nice day of spring, I killed my favorite koi by feeding him. I gave him a big old lunch of pellets and he gobbled them up as fast as possible, he was about 16″ long so he was a great size. His stomach hadn’t digested anything in 6 months, he ate a lot. It was quick, I had no idea what I had done. When he floated to the surface a few moments later he looked so stretched out It was easy to see what had happened. He basically burst from the inside. It was tragic. That’s a little about fish in cold water.
STOP FEEDING IF WATER TEMPERATURE HAS FALLEN BELOW 10 deg.