If you simply make sure that your pond is at least two feet deep, the proximity of the earth to the pond’s surface will not allow the latter to freeze any deeper than 8”. That leaves 16” for the fish to lounge around and basically hibernate over the winter. You do need to keep a hole in the ice (using a “floating heater”) to allow for the exchange of gasses (like oxygen). But other than that your fish will do just fine in the pond, all year round. Supplemental oxygen can also be supplied by running your waterfalls, adding a bubbler, or using the pump to churn the water near the surface.
Goldfish and koi hate wintertime more than we do. Neither species of fish are indigenous to North America, so in our colder climates, they merely “survive” winter. They don't flourish in it.
In the southern part of our country, the winters are pretty balmy and very little ice appears on the ponds. However, winter's effects on the fish seem to be the same whether the pond is merely icy, or completely iced over. Some important wintertime facts will help you guide your fish through winter and into a safe and healthy springtime.
There are certain things you should realize about winter so you can properly interpret certain events and conditions come spring.
Important Factoid #1
During the winter, the fish’s immune system is in a predominantly non-functional condition. In other words, their immune system is in hibernation.
Important Factoid #2
Temperature swings within the pond of 20° F or more are very stressful for the fish and moving water through a thin phase aids it in the gain or loss of heat. This is a simple statement with a lot of meaning. When you pour a cup of hot soup back and forth from one cup to another, you can rapidly cool it. In the same way, a waterfall can dissipate or pick up heat from the pond’s water.
In certain climates, such as in the Sierra Nevadas and other desert areas, air temperatures can be very warm by day and ice cold at night. This matters because if your waterfall runs around the clock, you could be warming the water by day, and super cooling it by night. Again, this is a geographical phenomenon, and may not apply to you but a simple pond thermometer could tell you for sure. The stress caused by fluctuating water temperatures makes the fish more vulnerable to infection.
To avoid this problem, some people run their waterfalls during the day to pick up valuable free heat, and turn the falls off (making sure to have some other form of submerged pond circulation for aeration) at night to spare that free energy and avoid super-cooling.
Important Factoid #3
Turning off your waterfall may spare heat loss at night, but it can also deprive fish of oxygen and circulation. It is important, especially if water temperatures are climbing, to always have some circulation in the pond to maintain sufficient aeration or oxygen exchange for the fish.
Important Factoid #4
Fish cannot freeze into a block of ice and survive. This is a wintertime factoid that should be destroyed once and for all. Many people see their fish in small ponds, “frozen” under a solid layer of ice. The fish are utterly motionless due to the cold. They perceive that the fish are frozen in the ice and so they say, “My fish were frozen solid and lived!” but this is not the case.
Important Factoid #5
Another common myth in this hobby is that fish are safer from parasites and pathogens, like bacterial infections, in the dead of winter because these “bugs” slow down, or even stop, in icy water. However, the opposite is true.
Parasites do not necessarily slow down in ice-cold water. In fact, certain species of flukes are actually more active in the icy water of winter, and species of ich, trichodina, and costia are also busy at work in icy water. It’s an important fact that the fish can be more heavily infected with parasites in winter than any other time of the year.
Becoming familiar with these facts will give you the understanding to help your fish have a restful winter and a healthy and active spring next year!