Having an area free of ice is crucial for koi ponds or ponds with fish to allow proper gas exchange. A pond is essentially dormant during cold winter months when biological activity is at a minimum. However, this does not mean there is no biological activity.
Fish respiration and metabolic processes still occur, steadily releasing waste products while using up oxygen. Without an opening in the ice, not only do harmful gasses such as carbon dioxide remain trapped under the ice but also the supply of dissolved oxygen is depleted. If this condition is allowed to progress and oxygen is not replenished, the fish are unable to survive the toxic environment and fall victim to what is commonly referred to as "Winter Kill."