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Should You Keep Your Koi Pond Filter On or Off in Winter?

Many pond owners and koi keepers believe that filters should be left on over winter. In this blog post, we will look at what a filter does and why this is often not the case.

Let us first consider what a filter is actually for. A lot of fish keepers, of both pond and aquarium fish, know that they need a filter but often do not know why. It is important to know exactly what each piece of equipment you are using does and roughly how it works. Therefore, if your equipment fails for any reason, you are more likely to be able to fix it, and when you need to buy anything, you can make the most educated choices.

How does a filter work?

First and foremost, a filter removes dirt and debris from the pond water by trapping it with the filter media. A filter does a lot more than this though! A pond filter usually has many stages to keep your pond water safe and healthy for your fish and also looking lovely and clean for viewers.

The first stage in a filter is the mechanical filtration. This is when the filter physically collects debris from the water and holds it in the filter itself. In an aquarium filter, this is usually done by a sponge or a polystyrene wool that traps the debris and prevents it from going further in the system, in the same way a sieve or a cheesecloth would be used in a kitchen. In a pond filter, sometimes the mechanical filtration is done the same way as in an aquarium filter, but often more interesting methods are used due to the space required for such filtration. For example, a Nexus filter, like the ones we use at Keruto Koi, has a round design to create a vortex in the middle of the filter, where debris and solid materials settle.

A Nexus 320 filter like the ones here at Keruto Koi that run all year round.

The next stage in a filter is the chemical filtration. As the name suggests, this type of filtration uses chemicals to purify the water by removing the impurities. The most common chemical used for this filtration is carbon in the form of either charcoal or activated carbon. The carbon becomes very porous when it gets wet and therefore has a massive surface area that allows any dissolved pollutants to be absorbed. Chemical filtration is great for removing any pollutants that end up in your water as well as organic waste produced by the fish. However, it has no effect on ammonia, nitrite and nitrate which still need to be addressed.

The final stage of filtration is the biological filtration. Biological medias, such as K1 media, are a fantastic home for beneficial bacteria. These bacteria live in the filter and have lots of roles to benefit your pond. Some of the bacteria break down the ammonia, nitrate, and nitrite in your pond water and other bacteria cycle nutrients allowing good nutrients back into the water while retaining the waste. Still others help to oxidise the water, thereby making it healthier for both fish and plants.

Not every filter will have all three stages of filtration. Most external aquarium filters will have all three while internal aquarium filters will usually skip chemical filtration to save on space and keep the filter as small as possible. When it comes to pond filters, the amount of filter media required for each stage can be massive so many pond filters will leave out one of the stages. A pond usually follows a more natural biological cycle than an aquarium so the pollutants that would be collected by the chemical filtration are rarely introduced to the water at all. Therefore, most pond filters will only use mechanical filtration and biological filtration.

So, what happens in the winter?

When the temperatures fall, the koi go into a type of hibernation called torpor. In this state, they will eat very little, mostly snacking on the organic material in the pond, such as algae. Therefore, the amount of waste produced is greatly reduced during this time, so some pond owners use this time to turn their filters off and "give them a break" over winter. Well, this may not be the best idea!

The main reason to keep your filter on during winter is due to the presence of the beneficial bacteria in the biological filtration. Turning the filter off will cause these bacteria to die off which will mean that when you turn your filter back on in spring, the filter will not be able to work effectively until the bacteria colony has built back up. This could take a long time, and, in the meantime, your fish will be much less healthy and more prone to diseases and illnesses which could kill them.

In addition to this, the movement of the water throughout the pond and the filter will help to prevent your pond from freezing over. A layer of ice on the top of the pond can be very dangerous for koi as it prevents the harmful carbon dioxide from leaving the pond and also blocks the helpful oxygen from entering your pond. If your pond does freeze over, there are actions you can take – have a look at our blog post ‘How to Prepare Your Pond for Winter’ here: for more information.

Finally, the filter will continue cleaning the water and clearing it of waste and debris. While this is required less in the winter than in other seasons, if the filter is not on, you can still get a large amount of toxic matter building up in the pond which can be harmful to your fish and leave you a massive cleaning job in spring before you are able to enjoy your pond again.

Overall, keeping your filter on over winter reduces the amount of work you have to put in once the fish come out of hibernation in spring. If you take care to prepare your pond for the winter and leave your filter on, your fish will arrive in spring looking happy and healthy and your pond will be ready for you to enjoy straight away!

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