Winter Pond Maintenance

To a certain degree, a laissez-faire approach with our pond may be justifiable in the winter, as there is little that fish require from us over this fallow period. However, there are a few simple measures we can take to ensure that, even after the harshest of winters, our fish are there to greet us in the springtime.

What actually goes on in a pond through the winter?


The vast majority of pond fish (goldfish, koi, comets, shubunkins and orfe) are related to the carp and have been overwintering in our climate very successfully for hundreds of years.

Fish are cold blooded, which means that they take their body temperature from that of the surrounding environment. When it is warm, they are active, feeding, growing and producing the waste to match. The reverse is true as the water temperature drops. As long as the drop in water temperature is gradual as it usually is between summer and winter, lagging slightly behind the reducing air temperature, then our fish should adapt to what for them is a quite natural phenomenon. They have no choice in the matter and respond to cooling water by a drop in activity as their metabolism slows right down.

A carp’s physiology is geared to overwintering and this physiology has been inherited by their ornamental cousins. The shortening daylength as winter approaches causes pond fish to prepare for winter before it arrives, feeding in excess (whenever possible) to store sufficient energy to survive the winter. The onset of cold weather may come as a shock to us, but our pondfish, unbeknown to us, have been preparing for this testing period for months.

As soon as the water temperature drops below 8oC, fish become relatively inactive, drop to the bottom of the pond, not to feed again until the temperature rises above 8oC in the spring. During this lengthy period of inactivity, we can intervene to ensure that any risks to their health and well being are reduced.

The Pond.

Ideally, a pond should be at least 3 feet deep. This provides pond fish with sufficient depth of water in which to be insulated from the freezing air at the pond’s surface.

A remarkable phenomenon occurs to water when it cools. Usually, the density of a material increases as it gets colder and this is true for water, but only down to 4 degrees C. Between 4 degrees C and freezing, the water actually gets less dense and floats above the denser 4-degree water, eventually to form ice if the air temperature drops below freezing. Our pondfish take full advantage of this phenomenon by sitting on the pond bottom. In very still and deeper water, quite a stable boundary can form between the less dense 0-4 degrees C water and the denser warmer water.

Water quality (and particularly clarity) is likely to be at its best in a pond during the winter. Reduced feeding and metabolic rates of ‘hibernating’ pond fish mean that ammonia production is likely to be negligible in a wintering pond. This will not burden a pond filter (which is also relatively inactive at these temperatures) and the fish are not likely to experience ammonia or nitrite problems.

Pond water is also likely to be at its clearest in winter as the cold water retards the growth of suspended single celled algae that can cause green water. The cold water will also hold relatively high levels of dissolved oxygen and as all of the oxygen consuming organisms (bacteria, protozoa and fish) are metabolising at a reduced rate, their need for oxygen is also reduced.

Pump and Filter.

As fish are excreting far less ammonia than usual, you may be able to consider whether the pump and filter system is required through the coldest winter months. The main reason for keeping the pump and filter running is to retain the maturity of the filter system ready for the start of the next season.

Circulating water in a winter pond can actually have a detrimental effect on fish health. Fish settle in the deeper warmer layers of the pond, but a pump can mix cooler surface water with the warmer water that fish are using as their sanctuary. Pumping water through a waterfall or water feature will chill the water down to the freezing air temperatures and should be avoided for a fishes’ well being.

If you opt to turn off the pump and filter, ensure that you empty all pipework to prevent breakages due to the expansion of ice. Also, turn the system back on in late February to help the filter to mature prior to fish regaining their appetites in spring.

Autumn Leaf-fall

Although water quality is likely to be at its best in winter, it can deteriorate if your pond is prone to collecting substantial amounts of leaf matter. Even at low temperatures, leaf matter will decompose, upsetting the balance of the pond. A simple pond net, stretched tightly across the pond will prevent leafs from falling into the water. A fine-mesh hand net should be used to remove any leaf matter and debris that has already accumulated. Do not be too concerned if a shallow layer of silt still forms on the pond bottom as fish can benefit from settling in a soft substrate over winter.


Ice in itself does not pose a direct threat to fish health, but can cause problems indirectly in a number of ways.

1. It prevents gas exchange between the water and air, causing a possible build up of toxic gases in the water. This is particularly a problem in a more mature pond that may have accumulated a substantial amount of settled organic matter.

2. Breaking the ice using a brick or hammer will cause fish severe stress through the extreme sound waves that are transmitted through the water. Ice should be melted using hot water and a permanent hole in the ice achieved using a low wattage immersion heater.

Overwintering Dos + Don’ts


1 Prevent ice from fully covering the pond

2 Cover the pond with a fine mesh net to stop leafs entering the pond and settling on the bottom.

3 Treat the pond with a general anti-bacterial treatment in the first warm days of activity in spring.


1 Panic. Provided you have fed your pond fish well over the summer, they will have reserves to overwinter. Research has shown that pond fish can live for 150+ days without food over winter before encountering real problems.

2 Smash ice if it forms. Use a hot saucepan or kettle and keep it ice-free with a 100w pond heater.

3 Leave aeration on as it will mix all water depths causing the water to chill quicker.

4 Feed in isolated sunny spells, even when encouraged to by fish activity. Particularly resist feeding in months of November to February, as prolonged cold periods will almost certainly return during these months.


1 Put a small pond heater on a thermostatic control to come on during a frost. Fit it and forget it.

2 Choose a black fine mesh net to prevent leaves falling into the pond. Not as visually obtrusive as a green net.

Winter Pond Information Centre: Overcome ice, frozen ponds and freezing weather safely.

Kill blanketweed and string algae.