How to handle a sudden drop in the water temperature of your koi pond
Whether it's because of climate change or not, every country in the world experiences unpredictable and unseasonal weather from time to time. The implications for anyone who keeps koi or goldfish, is that the water temperature of their pond water might drop dramatically long before winter arrives.
Like many creatures, fish will hibernate in winter, which is how they survive. But while they will tend to move to the bottom of the pond in the event of an unseasonal cold snap, they will usually return to the top for food when the weather improves. Generally people make more effort to consider the needs of koi as opposed to goldfish. This is because koi are hugely more attractive – and considerably more expensive than the common goldfish. Koi also grow much larger than goldfish and are even bred for competition internationally. How fish behave when the water temperature drops
All pond fish, including koi and goldfish, are poikilothermic, which means that their body temperature is variable, and usually slightly higher than then the temperature of its environment – in this case the water temperature of the pond water.
When the pond temperature drops, koi become less active. So don't be alarmed if they don't come inquisitively to the surface of the pond as they always do during summer. In cold weather they will instinctively move to the lower depths of the pond – which in icy winter weather is the warmest part of the pond water. What happens is that when water gets very cold it contracts and becomes less dense. But because the maximum density of water is 4 degrees C, as it gets colder than 4 degrees C the cold, less dense water rises.
Even if winter hasn't arrived, a cold spell may result in fish exhibiting typical winter behaviour. They also stop feeding during the cold winter months, and may not want to eat if fed during a cold spell. A good rule of thumb is that if koi don't come to the surface for food, don't feed them.
How to prepare for a drop in water temperature
Generally ponds and their surroundings, like gardens in general, show distinct seasonal changes. Because of this it is imperative to follow a seasonal approach to pond water care. For example, in autumn plants tend to die back and there will be less natural shelter for your fish. This is one of the primary reasons that koi ponds should never be a uniform depth. Ideally have a shallow shelf around the edges where they can bask in sunny weather, and deeper areas where they can retreat to when they hibernate. It is good practise to use rocks within ponds for shelter from predators, and for fish to spawn. Another effective exercise is to lay a series of empty pipes on the bottom of the pool where the koi can hide if they wish. During autumn it is essential for koi to be fed plenty of food to fatten them up for winter when they generally don't eat.
If you feed during a cold spell, you are likely to find that the fish ignore the food. If this happens, scoop the food out quickly, otherwise it will remain in the water and decompose. Of course if you have installed a pump and filter (which most serious koi keepers do), then any excess food should be filtered out. If you live in a part of the world where pond water tends to freeze over in winter, a koi pond heater is certainly worth considering. Not only can you use a heater to melt the ice and maintain a higher water temperature during winter, but you can also use it in the event of an unseasonal cold snap.