As carp fish, koi are able to adapt to numerous conditions. This also includes the ability to react to the ambient temperature and to control life functions accordingly. Nevertheless, strong fluctuations and extremely high or extremely low values ​​should be avoided – because, in the long run, they lead to stress, damage or even death of the animals.

The optimal housing conditions for them are 23-25 ​​° C. In this temperature range, the fish develop or retain the greatest resistance to parasite infestation, infectious diseases and skin or digestive disorders. In addition, metabolism, growth and spawning behaviour and the development of eggs and larvae are optimally adapted to this relatively high level of heat.

If the temperature suddenly drops by more than 10 ° C, most Koi react with a clearly visible change in the complexion and/or lasting damage to the digestive tract, which can even lead to death sooner or later. The lowering to the 4-6 ° C, which is usual in winter, must therefore take place gradually; As a rule, the time required for this is at least 23 days. For specimens that are known to be sensitive, it is sometimes even twice as long.

In contrast, the animals can cope with a rise in water temperature and only need a few hours to adapt to a warmer environment. However, the metabolism skyrockets and uses up to 50% of the fat deposits. If Koi do not have enough serpentine reserves, they die as a result of a lack of energy. In order to avoid stress or health hazards for the animals, the difference between day and night temperature should not be more than 2-3 ° C. 

However, caution is advised if this results in values ​​between 8 and 12 ° C, as the Koi’s immune system rests in this area; harmful bacteria, fungi and/or parasites are still active. In addition, the digestive system still works at these temperatures, so that there are excretions and an increase in nitrogen values ​​in the water.