Preparing Outdoor Ponds for Winter

Depth and Insulation

For fish to overwinter safely outdoors, Aquascape recommends a pond depth of at least two feet. This depth prevents the pond from freezing solid, as the earth’s warmth helps insulate the lower layers. This leaves room for fish to remain in a semi-dormant state near the pond’s bottom, where they’re protected from the coldest temperatures.

Gas Exchange and Oxygenation

Maintaining an opening in the ice is essential for healthy gas exchange, as it prevents toxic gases like ammonia and carbon dioxide from accumulating beneath the ice. This can be achieved with a floating de-icer, which creates a hole in the ice and allows gases to escape. Alternatively, a pond aerator or small pump placed on a shallow ledge (not at the pond's bottom, to avoid disturbing warmer water) can keep part of the pond ice-free and provide oxygen.

Waterfall Considerations

In regions with significant day-night temperature shifts, running a waterfall continuously may cause unnecessary heat loss at night. Some pond owners choose to run the waterfall during the day for mild heating and switch to aeration at night to conserve warmth. However, it’s crucial to ensure some form of water circulation is always present, as stagnant water in cold weather can lead to oxygen depletion.

Understanding Fish Dormancy

When water temperatures drop below 50°F (10°C), koi and goldfish enter a state of dormancy. During this period, they require no food, as their metabolisms slow significantly. Feeding fish in cold water can cause digestive issues since they cannot process food effectively at low temperatures.

Additional Considerations

Parasite Activity

It’s a common misconception that parasites become inactive in icy water. In fact, some parasites, like flukes, remain active and may even pose a greater threat during winter. Maintaining clean water and stable winter conditions can help minimize these risks.

When to Consider Moving Fish Indoors

In extremely harsh climates or for ponds that lack the necessary depth and insulation, moving fish indoors can be a safer choice. An indoor setup allows for stable temperature, oxygen, and monitoring of water quality, providing fish with a controlled environment until spring.