I wonder what my fish are thinking under that ice. I live in the north in the USA, and for about 2-3 months a year the pond is covered by ice. I keep a small hole open for the gas exchange, but I can’t see my fish. What is going on under there? Did you ever wonder?

Well, that would depend upon how carefully you cleaned their pond before the cold weather set in. There are a few scenarios that might be happening under the ice, and hopefully everybody is okay down there.

If the pond was not cleaned, what is probably going on is parasites. Parasites do not sleep or hibernate in cold temperatures. They proliferate in the mulm and debris on the bottom of ponds. In the cold, pond water will be clearest and has the most dissolved oxygen. The fish metabolism will shut down at around 45◦F whereas any parasites in the pond will continue to go about their business unaffected by the low temperatures. Eventually, the parasites will begin feeding on your fish but the fish won’t notice until the temperatures begin to warm up and their feeling returns. (So, that’s why all the flashing and jumping is seen in spring.)

If the pond was cleaned well, the fish should be sleeping and comfortable. But, another thing might be happening: are they sleeping in their own “stew”? Meaning, did you shut down the bottom drain in order to keep the bottom a slight bit warmer through the winter months? Well, there are different schools of thought on this. Some want a mid-water winter draw and return while others insist upon leaving the bottom drain open to continuously clean the area in which the fish are living. I lean to leaving the bottom drain open and functioning normally. I leave my filters on year round. Although bacteria in the filter do cease to perform below a certain point, they will be there come spring. And I want the mechanical filtration to continue. Any leaves or fish urine in the water, or ammonia from the fish’s gills, is not what I want my fish to stay in. I want them in clean water, whether the water is warm or cold. This also will remove more parasites and places for parasites to live.

One more thing that I can think of, under the ice, could be happening. The fish could be in serious trouble if that hole in the ice layer closes up. The gases that build up would be carbon dioxide, and when it cannot be released into the air, can become lethal. It will actually become carbon monoxide without the influx of new oxygen.

So, please keep that hole in the ice open, keep the pond clean, and keep the filters going this winter if you live in a cold weather region!