Alan’s Book

Chapter 11 The Four Season Koi

Carp, the ancestors of koi, originated in the Caspian Sea area where winter temperatures rarely fall below 14C and summer temperatures can reach temperatures of 28C plus. Although they have been transported all over the world their immune systems are still adapted to the temperatures of the areas where they evolved; due to this when winter temperatures fall below 14C their immune systems are not fully active.

As seasons change there are changes in temperature, the length of daylight hours and the intensity of the sunshine. The natural environment, both terrestrial and aquatic, changes in line with the seasons.

In a natural environment food supplies are self generating and self limiting; plant nutrients (minerals) such as nitrates and phosphates in the water provide a food source for microscopic plants and plankton. Sunlight provides an energy source for them to grow.


Springtime is accepted as being the time of awakening in the natural world. Daylight hours lengthen and temperatures begin to rise. Sunlight and temperature stimulate the growth of living creatures including the aquatic organisms upon which koi feed. This presents an ideal time for koi to build up their residual food supplies and increase the eggs and sperm which will help to guarantee the future of their race.


The main function of any animal is to reproduce; ideally young would be produced when a plentiful supply of food is available such that they can grow.

By early summer the koi should have improved their condition after the relatively lean winter period when little food is normally available for them and should be ready for spawning. Water temperatures will have increased signalling to the koi that eggs will deposited into water at a suitable temperature for them to develop. The natural food production will be in full swing maximizing the availability of food for the fry.

Providing there is sufficient food available the fry grow quickly and enjoy nature’s rich bounty. The adult koi also enjoy the abundant food which is available during this period to both replace the energies expended during spawning and to boost their growth.


The reduction of daylight hours and falling water temperatures signals to the koi that the lean period of winter is approaching and that reserves of food should be stored in readiness for this period. Nature has provided for such a time and the aquatic food larder presents a host of high quality food such as snails and water shrimp upon which the koi will feed heavily during this period, and by doing this they build up reserves for the winter ahead.


Daylight hours are at there shortest during wintertime and water temperatures are at their lowest.

The metabolism of the koi and other pond inhabitants is directly proportional to the water temperature in their normal range, so the koi and other inhabitants will naturally slow down during the winter period conserving their supplies of energy from their stored food reserves (fat).

Some food may be available, but during wintertime with low sunlight levels the natural food production in the pond will be at a minimum.

It should be appreciated from the above that the distinct seasons encourage distinct behaviour patterns in koi under natural conditions so it would make sense to try and replicate these condition in our koi ponds; daylight, temperature and food are the main considerations.

There is little we can do about the length of daylight hours our ponds experience, but we can control the food supplied to the koi and if necessary, the pond temperature.

Pond Temperature

Where you live will determine the natural range of pond temperatures throughout the seasons. Winter temperatures of 14C are regarded by some as the minimum a koi should experience. In practice this temperature can be very expensive to maintain and temperatures of 8C minimum for 6-8 weeks during January and February seem to present few problems providing koi are correctly housed and adequately prepared for this cold period during the autumn management.

Summer temperatures can be as important as those of wintertime and extending the summer temperatures in the U.K. up to 26C for several weeks in the summertime is not an expensive exercise. At this temperature the koi’s immune system can quickly repair minor damage and scale loss ensuring positive healing before the winter period.


Unlike terrestrial animals koi do not have to maintain a constant body temperature; body temperature maintenance uses large quantities of the energy we acquire from our food. As koi do not use energy from food to maintain a constant body temperature, their requirements for body repair/maintenance is lower and the majority of the energy is used for growth and reproduction.

Whereas nature controls the food availability in natural ponds in a structured manner by natural balanced cycles of sunlight and temperature, we have to judge the correct amounts of food by watching water parameters and our koi, who require only small amounts of food to survive.

The availability of food during the seasons as explained above allows us a good guide as to how we should feed the koi in our ponds.

Increased light feeding as temperatures rise in springtime will replicate the way food production naturally occurs, and will allow the filter system time to expand its colonies of bacteria to match the increase in ammonia that the increased metabolism and food consumption will produce. It is very important that pond parameters are monitored on a regular basis, but much more important at this time of year when the eagerness to feed our koi can lead to undesirable levels of ammonia and nitrite in the pond water.

Undesirable levels of ammonia and nitrite can compromise the koi’s immune system and as the immune response is related to the koi’s metabolism it is unlikely to be fully operational at this temperatures prevailing at this time of the year.

In some ponds low levels of parasites may take advantage of any weakness in the koi’s immune system defences and increase accordingly. This can lead to bacterial infections such as ulcers if the integrity of the skin is breached.

Springtime conditions in koi ponds require careful monitoring such that the koi and all of the pond inhabitants progress towards the summer period without undue problems.

Far more koi die or become damaged through bacterial and parasite problems because they are overfed rather than underfed!

Once temperatures rise above 14-15C in late spring the koi will become more active and the activity of the filter will increase accordingly.

In an ideal world temperatures would increase slowly and steadily upwards at this time of the year however British climates are notoriously variable and warm and cold spells in the weather can mean increases and falls in the pond water temperature. It is at times such as this that a heating system can provide massive benefits by stabilising the pond temperature. Repeated increases and decreases in temperature are highly undesirable and can adversely affect our koi.

Once early summer arrives temperature fluctuations are usually, but not always less prominent however a heating system is a convenient means of setting a minimum temperature for the time of year and in the U.K. a temperature of 18C would be an acceptable one to maintain.

Feeding can be reasonably liberal at this temperature within the limits imposed by the filtration system.

Where possible summer temperatures should be increased to around 24C for several weeks and this will allow the koi’s immune system to deal with any minor health problems as explained previously.

By late summer and early autumn the koi should be very active and eager for food. It is at this time of year that food should be available in generous quantities. I feed a high proportion of whole prawns and mussels at this time in order to prepare the koi for the lean winter months ahead.

The temperature of the pond will gradually decline as the summer proceeds to autumn; once water temperatures fall to around 10C in December, feeding should be reduced.

My koi are all housed indoors and temperatures do not fall below 11-12C in wintertime, but during but in December I reduce the food until by Xmas they are fed only very occasionally and only with small amounts of food, mainly mussels and whole prawns including the shells.

This regime is followed during January and February, and during March the food is gradually increased once again as spring conditions reoccur.

The annual cycle is based upon light levels and temperatures which give signals to our koi with respect to reproduction. Relatively high temperatures and feeding levels in wintertime can encourage high levels of eggs to be produced and the fat reserves remain unused which can lead to problems of obesity.

There will be those readers who prefer to keep their koi at relatively high temperatures throughout the year believing that this is good for them. Perhaps this chapter may encourage such keepers to take a slightly different approach to how they keep their koi; reducing/withholding food during the winter period would be one aspect of their management which should prove beneficial to the health of the koi in the long-term.

They above procedures are recommended for koi which are two year olds and over. Koi are often bought as “tosai” (current year koi less than one year old); at this age they will be sexually immature and the females are unlikely to build up eggs in response to the seasonal changes. These young koi are best managed by keeping them at a higher temperature during their first winter with the provision of adequate amounts of suitable food to encourage growth.

Koi Food

Koi pellets are available in a variety of sizes and protein levels; some are recommended for certain temperatures and others are “multi-seasonal” meaning they may be fed all year round.

Generally speaking young koi require higher protein levels and older koi require lower levels. Feeding pellets with high protein levels will mean more work for the filter system to cope with due to the increased ammonia which these pellets produce; higher protein levels are best fed at temperatures above 18C

It is very important that koi pellets are stored correctly and are used when they are as fresh as possible; a sealed container which should be closed after use will help exclude damp air which can encourage fungal growth on the pellets. Fungal growth can result in toxins being produced in the pellets.

The vitamin most likely to deficient in koi pellets is vitamin C so a wise precaution would be to occasionally add vitamin C to dry pellets by using Vitamin C effervescent tablets sold for human use in health stores and supermarkets. Dissolving one 1000 mg tablet in a small amount of water and adding this to dry pellets allows the pellets to absorb the water containing the vitamin C. After suitable drying the pellets may be fed to the koi; only feed what the koi will immediately eat as the vitamin C and other valuable nutrients contained in the pellets will be lost if the pellets become water logged.

Vitamin C is associated with wound healing in koi and can be beneficial to koi when they are recovering from injuries. I tend to use these pellets as a food supplement once per week or when quarantining new or injured koi and they are readily accepted by all of them.

Oranges can be cut into halves and the koi will suck out the fruit leaving clean skins as an indication of how they are appreciated; these may be offered on a regular basis as they provide exercise and stimulation, however the concentrated tablets ensure a good level of this essential vitamin.

Cooked mussels and whole prawns in their shells are a welcome addition to a koi’s diet as are fresh greens such as cabbage and lettuce with occasional amounts of spinach leaf. As with the provision of oranges, these additional foods keep the koi active.

Feeding whole mussels and prawns means that the koi obtains micro nutrients which these creatures have used for food and the prawn shells may contribute valuable calcium to the diet especially in soft water areas.

There are many ways to keep koi but from the onset of this book I have tried to give advice on how to keep them in conditions which are as close to the optimum conditions as is practically possible.

If we are aware of the natural seasonal variations of temperature and food availability and follow similar practices in our ponds as explained in this chapter, it may take us a step further towards providing the illusive optimum conditions for our koi.

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