Why can Koi Change Color and Pattern?

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You may have noticed many times when keeping koi fish that the colors and patterns they showed when you bought them are not the same after a year or so. This happens for a variety of reasons and today we will discuss the main ones.

The first and foremost reason is genetics. Each fish has a combination of dominant and recessive genes from its parents that determine the colors and markings it will display as it grows. For example, a Kohaku fish will have genes corresponding to the shiroji (white) base color and hi (red) markings, while a Showa fish will have genes corresponding to the sumi (black) base color and hi and shiroji markings (red and white respectively).

Some koi varieties can change drastically throughout their life, making it difficult to judge a young koi and predict what it will look like as it ages and grows. Take the Showa Doitsu Be Koi for example. Our Showa Doitsu at 25cm was bright orange with little sumi (black), but the adult Showa Doitsu at 40cm is a deep red with more sumi, making the pattern of the adult Showa Doitsu Koi much different from when it was small.

This is an extreme example, most koi will not change much during their lifetime and there are often indicators in the patterns of young fish to give you a clue as to the patterns of the adult fish. For example, you may see some young koi with beautiful blue markings. This is usually due to the darker black pigmentation under the skin meaning that as the fish ages the blue markings tend to become black markings.

The next reason why fish can change color is because of their diet. A good, healthy diet with the right vitamins and nutrients to support vibrant colors will result in happy, healthy looking fish with beautiful, clear markings and color differences. If your koi are fed a poor diet, it can result in your fish looking more watered down and faded with little difference between the different colors and markings on your koi.

Another common reason why koi appear faded and pale is stress, particularly stressful water conditions. Other stressors, such as transportation, can cause koi to lose their color, but they usually recover quickly in these cases. So, if your koi arrive looking a little more faded and pale than you expected, rehome them and they should regain their beautiful, vibrant colors and markings within a few days.

Stressful water conditions can include dirty or oxygen-depleted water. We recommend that you use a filtration system appropriate for your setup and regularly check your water quality with a test kit by measuring the amount of ammonia and pH levels in your pond. It is also important that the water you put into your pond is suitable for the fish you will be keeping by using a dechlorinator or other method to remove harmful chemicals and minerals.

The final potential reason for your koi’s color change has to do with the amount of sun exposure your fish are receiving. This is actually rarely seen in koi except during the winter months. Since your koi will be exposed to less sunlight during the winter months, they may appear a little pale and less vibrant with little difference between the markings in early spring. However, as spring and into summer approaches, your koi will quickly lighten back up to their normal, healthy, vibrant color. This is very similar to how we tan in the summer due to sun exposure but become paler in the winter due to less sun exposure.

Koi fish can change color and pattern quite a bit over their lifetime. For some varieties, you may see a slight change from juvenile to adult size, for others the changes can be quite drastic. A big part of what makes a koi breeder successful is their ability to predict how a particular koi’s pattern will change over its lifetime. This is never easy, but some koi breeders have a very keen eye and consistently produce fantastic quality koi for us to enjoy and share with you.

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