The steps that you’ll need to take to keep your pond fish healthy in winter will vary some depending on a lot of factors: the size and depth of your pond, the equipment you use, what kind of fish you have and, most of all, how cold it gets where your pond is. There are a few rules, though, that should help out any pond owner to keep their fish healthy throughout the cold months.

• Get the water balanced before the weather turns cold.

Obviously, you should be trying to keep your water as well-balanced as possible all year round. However, I’m a big believer in putting in a couple of extra partial water changes when the weather starts to get cooler. This helps to get rid of some of the byproducts of your pond’s metabolism (such as nitrates), giving your fish healthier water through the cold months.

• Keep your water balanced through colder weather

I’m a big advocate of adding cold water bacteria to your pond when the mercury drops. The metabolisms of the bacteria in your pond slow and even stop in winter, meaning that they’re doing less cleaning of your pond. Cold water bacteria will keep metabolizing waste in your pond in colder weather.

• Change your feeding schedule
  

As the water gets cooler, the metabolisms of pretty much everything in your pond slow down. This is just as true of your fish. Slower metabolisms mean that it’s more difficult for your fish to digest what they eat, and this can cause problems. When the water gets below 55 degrees F, I strongly recommend switching to a specialized cold water fish food. This is less rich and easier to digest, and is much healthier for your fish. If your water drops below 42 degrees, stop feeding the fish. Their metabolisms get very slow at that point, and adding food to the water will just be throwing your water out of balance.

• Keep your pond surface open
  

If your pond freezes completely over, gas exchange between the air and the water stops. That is very bad for your fish. Trying to break a hole through the ice on the surface of your pond can be even more dangerous for your fish, though, as the shock waves through the virtually incompressible water will very likely injure or even kill your fish. The better solution is to keep your pond from icing over completely in the first place. In much of the US the bubbles from an aerator can actually be enough to keep the pond from freezing completely, and an aerator is something that you should probably have in your pond anyway. If you live in a particularly cold part of the country, you might need a dedicated de-icer.

• Keep your water clean before it’s a problem
  

Get that net up, and get it up early. Once the leaves start falling, you’ve probably waited too long, and now you’ll have a pond full of things that will spend the winter decomposing, making the water less healthy for your fish.