What Happens To My Fish In The Winter?
Goldfish and Koi are amazingly resilient. As the water temperature drops, their metabolism slows down proportionately. They need less food and less oxygen to survive and enter a lethargic period. During this time they are still moderately active, but are expending much less energy than normal.
Because your fish are using less energy, they require less food. Generally speaking, when the water temperature drops to 55 degrees it is best to stop feeding your fish altogether. They will find enough organic matter in an established pond to satisfy them. Decaying plant material and algae provide an adequate diet in most cases during the winter months.
The #1 killer of fish in ponds is overfeeding. They don't get fat and have a heart attack, it's much worse. The more fish eat, the more ammonia they produce. With the pond in a dormant winter stage, the nitrosomona and nitrobactr bacteria are at their lowest numbers. Without the bacterial colony to process the ammonia into nitrate, the free ammonia in the pond water strips the slime coat off the fish and suppresses their immune system, thus setting them up for a viral, bacterial or fungal infection which may, at this point, be lethal. During the warm season, feeding your fish a high quality diet sends them into the winter with a fully functioning immune system and little need for food during the cold winter.
In a properly designed water garden or koi pond, the water should never freeze solidly in our area. At the very worst, we normally have no more than four inches of ice on top of the pond. The fish are snoozing comfortably on the warmer bottom of the pond.
If your pond freezes- Never Break the Ice! Hitting the ice with a solid object sends shock waves throughout the pond often rupturing air bladders in the fish.
You really don't have to worry about your fish in the winter. Feed them a high quality diet during the summer, and they should be fine in the spring.