There are a number of reasons why fish may pass away during the winter season. This is especially true for seasons that are particularly long or intense. Lack of oxygen, overabundance of toxic chemicals, starvation, or simply freezing to death are all options that may kill a fish during the winter. Make sure that your fish are suited to the temperatures in your region. Additionally, make sure that you're preparing your fish for winter months by encouraging them to enter dormancy. This includes reducing feedings as the temperatures drop below 70 degrees, and halting them entirely once the water is below 50 degrees.
Toxic gases build up in the water if they are trapped underneath a layer of ice. Waste and decomposing organic material can lead to increased levels of sulfur, ammonia, and carbon dioxide. These can all poison or suffocate fish if there is no way for the gases to escape.
