Pond Water Temperature Changes:
Fish Pond Water Temperature in Autumn

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Autumn in the USA and the UK is traditionally one of the most unpredictable times of year with regards to the weather. Indian summers occur where sweltering hot days in early October can quickly be followed by a sharp drop in temperature, resulting in extremely cold water in our koi ponds.

Naturally, these sharp changes in pond water temperature can be a cause of concern for many koi keepers.

Cold Water in our Ponds is Not that Big a Deal

Fortunately though, despite what you may hear to the contrary, cold water in our ponds is not that big a deal.

Our beloved koi and goldfish are actually surprisingly tough when it comes to living with extremes of water temperature, and are as comfortable in cold water ponds as they are in warm water ponds.

In fact, they actually need the water temperature to drop, as their breeding cycle is linked to the changing seasons – therefore, they expect a colder period to arrive, when they will be much less active than in summer months.

Koi and Goldfish Behavior in Cold Water: Koi and goldfish are known as poikilothermic creatures, in that the water temperature will govern their rate of activity and their metabolism.

Basically, what this means is that your koi will be a lot less active in cold water than they were in the warm water of the summer. You will also notice that they feed a lot less as the pond water gets colder, until such a time (usually when the pond’s water temperature drops below 10C/50F) as they stop eating altogether.

Should I Be Worried?

When this happens, you need to stop feeding your koi. You might be worried about this, but the simple fact is they are not going to eat any food you give them, so whatever you put into the pond water is only going to pollute it, thus affecting your koi’s health.

What you should do, is make sure that your koi or goldfish have been well fed while the pond water temperature is warmer. During the summer, make sure they have a very healthy, high-protein diet in order to accumulate enough reserves to last through the cold winter months when they won’t be eating.

As we enter the first days of autumn, you can begin to reduce the amount of food you give to your koi and goldfish, hopefully keeping track of their slowing metabolism. Give them less food as they begin to eat less.

Koi Hang Around the Bottom

You will also notice that your koi will begin hanging around at the bottom of the pond as winter approaches and the pond water temperature drops. This is because cold water tends to be near the surface, while at the bottom the water remains a little warmer. Because your koi try to stay in the pond’s warmest reaches as the water temperature lowers, you should be careful not to mix the warm water at the bottom with the cold water at the surface, as doing so will cause stress to goldfish and koi.

Things to do when Cold Water Arrives

There are a number of preparations you can make once the first cold snap of autumn arrives, in readiness for the coming winter. The following list of measures will give your koi and goldfish the best possible chance of surviving the winter water temperature drop.

1. Clean Filter

Clean out the water filter in your ponds.

2. Prevent Freezing

Look into methods for preventing your pond water from freezing over.

3. Apply Netting

Apply netting over your ponds, to prevent leaves falling into the pond water.

4. Remove Aeration

Remove aeration from the deepest part of your pond, as you should try not to unsettle this warmer water.