Winterizing a koi pond prepares the aquatic environment and its inhabitants for freezing weather. This preparation ensures the long-term health of the fish and prevents damage to the pond’s mechanical systems. Koi are cold-blooded animals whose survival relies on maintaining a stable, protected environment as their metabolism slows down for the winter. The goal is to create a safe refuge that supports the fish through their period of dormancy and preserves the pond’s structural integrity until spring.
Preparing the Pond Environment
Preparation must begin before water temperatures drop consistently below 50°F (10°C). The first step involves manually removing all organic debris, such as fallen leaves, submerged plant matter, and accumulated sludge from the pond bottom. If this material decomposes under ice, it generates toxic gases like hydrogen sulfide and carbon dioxide, which can be trapped and harm the dormant fish.
Aquatic plants require specific attention, as dying foliage contributes to the toxic load. Hardy marginal plants should be cut back to just above the water line, and tropical aquatic plants must be removed entirely before the first frost. After physical cleaning, check the pond’s water quality parameters, including pH, ammonia, and nitrate levels.
A partial water change, typically 25% of the total volume, should be performed to dilute accumulated toxins and replenish trace minerals before the final freeze. Pond depth is also a factor in fish survival. Koi require a minimum depth of 3 to 5 feet in colder climates to access a layer of water that remains consistently above the freezing point, usually around 39°F (4°C). This deeper water provides a stable thermal refuge for the fish.
Managing Filtration and Circulation Equipment
The main biological filtration system must be managed carefully because beneficial bacteria become inactive in colder temperatures. When the water temperature drops below 50°F (10°C), the main filter should be shut down completely. This prevents the death of bacterial colonies, which could release harmful toxins back into the water. This shutdown also applies to ultraviolet (UV) clarifiers and waterfall pumps, which are not designed to operate in freezing conditions.
All circulation pumps, including those used for waterfalls or external filters, must be removed from the pond to prevent damage from expanding ice. Pumps should be thoroughly cleaned of debris and stored in a frost-free location, submerged in a bucket of water. Keeping the pump seals submerged maintains lubrication and prevents them from drying out. This prevents failure when the equipment is restarted in the spring.
External filters, plumbing lines, and skimmers must be completely drained of water to avoid cracking when temperatures fall below freezing. Any external piping or fittings that cannot be removed should be disconnected and allowed to air dry. These steps safeguard against replacing equipment damaged by ice expansion.
Protecting Koi and Water Integrity
The most significant step for fish survival is adjusting the feeding schedule based on water temperature. As the water temperature falls below 68°F (20°C), the koi’s metabolism slows down, and their digestive system becomes less efficient. Owners should switch to an easily digestible, wheat-germ-based food during this transition period.
Feeding must stop entirely once the water temperature consistently drops below 50°F (10°C), as the fish enter a state of torpor. The koi’s digestive processes are too slow to process food, and undigested pellets remaining in their gut can rot, leading to severe bacterial infections. The fish rely on stored fat reserves to survive the winter. Do not feed them again until water temperatures consistently rise above 50°F in the spring.
Maintaining a clear opening in the surface ice is necessary to facilitate gas exchange between the water and the atmosphere. This opening allows oxygen to enter the water and toxic gases, such as carbon dioxide, to escape from beneath the ice seal. Failure to provide this exchange point can lead to a dangerous buildup of gases that may suffocate the fish in the dormant bottom layer.
A floating pond de-icer or a small air pump with an air stone placed near the surface can maintain this opening. Avoid placing aerating devices at the deepest point of the pond. This will mix the colder surface water with the warmer, more stable bottom layer where the koi reside. A forceful physical method to create an opening, such as striking the ice, should never be used, as the resulting shockwaves can cause internal trauma or death to the dormant fish.