Do Koi Fish Need Sunlight and Darkness?

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KoiPond0101 PetAquariums Do Koi Fish Need Sunlight and Darkness?

According to the APPA National Pet Owners Survey 2019-2020 pet fish rank 3rd among the total of 84.9 million households that own them in the US.

Starting The Koi Hobby: A Step By Step Guide…

Over 12 million households own fish as a pet and arguably Koi are the most favorite including me. We not only love our pet friends but also care for them. Every detail is important.

Like the question, “do Koi fish need sunlight and darkness?” It has been troubling me since I got the latest addition to my aquarium, my beautiful “Nishikigoi”. So, I started digging, and here is what I have found about the lighting for the Koi.

Do Koi Fish Need Sunlight and Darkness? Well, like almost all the natural living beings on earth Koi fish also need sunlight, at least some form of it direct or indirect, and darkness too. Ideally, it should be 8 up to 12 hours for the phases each. You see, nothing can be better for the koi than mimicking their natural environment including the light & darkness like any other pet for that matter.

Now, this is only the tip of the iceberg. A lot of other factors are there too. Especially, when it comes to lighting, some experts even say that it’s one of the most important aspects that truly completes your experience and the health & beauty of your aquarium with everything in it.

So, to fully enjoy the serenity, charm, and beauty as well as to keep our mesmerizing colorful friend healthy & happy, let’s keep reading.

The Necessity of the Koi for Sunlight and Darkness

The “Nishikigoi”, Japanese for Koi, first caught the eyes of pet fish lovers at the Tokyo exhibition in Japan in the early 1900s. As an excellent work of art, it has been admired all over the world ever since.

“Nishikigoi” literally translates into ’living jewel’ or ‘swimming jewel’ for their affable gentleness along with refined beauty. So, for the relaxing elegance of the Koi, a proper lighting system including sunlight and darkness is essential.

Here are the three main reasons why and how the koi need adequate sunlight and enough darkness. For the,

  • Koi fish’s health & comfort.Graceful color, color pattern, and color mutation.Overall ecosystem.

Koi Fish’s Health & Comfort

Adequate sunlight and enough darkness are critical for keeping the Koi comfortable. A comfortable Koi also means a happy Koi. Hence, the food intake remains balanced, therefore the growth as well which leads to good health.

Now, the sunlight which is adequate for the Koi is generally the indirect light. Although, it never hurts a bit of direct sunlight every once in a while. So, it is best if you have your aquarium or indoor fish tank placed in such a position in your house where it can get enough natural light throughout the day.

The natural sunlight helps the Koi to get closer to its original photoperiod. Hence, the closer the recreation of its natural environment gets the better the Koi tend to do in response.

In terms of duration, as we already know, it should ideally be 8 hours to no more than 12 hours per phase, especially the light. However, if the Koi remain in full brightness within a combination of natural sunlight and your artificial lighting system they should get at least 8 hours of full darkness if not more.

On the other hand, it’s the lighting that truly makes the Koi and the surrounding beautiful world under water shine. Hence, on the weekends or occasionally if you want to adore the breathtaking beauty of your precious pet without stressing, then you should use soft lighting followed by giving it a bit longer than the usual resting period in peaceful & silent darkness.

Lastly, keep the design of your aquarium or tank minimal, and don’t forget to put some places for your pet friend to hide. It will be a great gift for the koi if they need to take a bit of rest sometimes or let the stress out.

The Graceful Color, Color Pattern, and Color Mutation

One of the important criteria, if not the most, which makes the koi fish dazzling and stand out are their vivid & bright colors along with color patterns, and color mutations. The natural light of the Sun not only makes these colors rich and diverse but also keeps them vibrant. In addition to that, it seemingly stimulates Koi’s color pigment creation, mutation, and exhibition too.

These elegant swimmers show a wide range of shining color variations. There are around a whopping 200 breeds to choose from each with their unique style of coloration. Yet the glow and the charm they showcase are heavily influenced by the light they get, especially the natural sunlight among other things.

Along with the way they swim, different breeds of the “Nishikigoi” are mainly judged & distinguished, depending on the variety being observed, by the pattern they form, the scale of their body, and the combination of colors they have: pure white, red, black, orange, etc.

However, the pigments that are behind these multiple majestic colors can be classified only into two sets namely the black and the red pigment group. The combined role of both the group’s result in different, dark to bright, striking skin colors that get quickly induced by their environment’s light.

In an environment that is comparatively darker overall, the color tends to shift towards more of a black-pitched tone. On the other hand, if the environment is exposed to light for an exceeding period of time therefore for overall brighter, they are very likely to lose the vibrancy of their color.

So, at the end of the day, they do need light and they do need darkness. Only just the right amount in a balanced way and it is excellent if it comes naturally: from the sun and the night.

The Overall Ecosystem

The entire aquarium or your fish tank is a complex ecosystem in and of itself. So, one parameter naturally affects all the other and light & darkness is no different. Besides, on earth and for life as we know it, sunlight is the main source of energy anyway.

Hence, a lighting condition that includes the sunlight properly is bound to be best not only for the koi fish but also for the entire ecosystem as a whole. Not too much, not too little, just the perfect lighting cycle is also best for the algae the fish rely on for natural food. Not to mention the plants and bacteria, the Koi rely on too.

However, too much or frequent direct sunlight can also cause a rapid increase in the population of algae. Consequently reducing the visibility in the water even making the whole environment barely habitable for all other aquatic life in its extreme. So, it is very important to maintain balance.

Sunlight is ideal for most of the aquatic plans too for photosynthesis in the tank. If the plants thrive with proper photosynthesis while creating an excellent, clean aquatic environment so does the Koi. Speaking of the cleanliness of the water, it is very important for the Koi since they are freshwater fish.

Apart from the aquarium’s artificial filtration system, bacteria are the natural water cleaner that works best in natural light. They not only decompose the poop of the Koi but also remove nitrite as well as ammonia from the water. These chemical elements are highly toxic for the living jewel.

Thus, to cut the long story short, the Koi fish and the entire aquatic world around them depend on light and the best light is the natural light that comes from the Sun.

Natural vs. Artificial Lighting

Now that we know all about the necessity of light and darkness for our beloved pet fish, let’s talk about different lights. In an indoor aquarium or tank most often the artificial lighting system plays a crucial role along with natural lighting. Sometimes it is the main source of light of the aquarium.

If this is the case, the light should be chosen carefully. The PAR rating should be adequate for the Koi and other aquatic life in there. You should design the lighting system in such a way that you can easily control the intensity.

The timing & artificial light cycle of each phase should be in harmony with the natural cycle. Such as if you get enough natural light then the use of artificial lighting should be minimal accordingly. Likewise, if the lighting phase goes longer the intensity should be lower and an extended period of complete darkness should follow afterward.

Finally, in the case of a pond outdoors, the natural light is sufficient. You can add some underwater lights too but not too much. Additionally, to balance the outdoor direct sun, it is best to have some shade. The outdoor shading can come from a tree nearby. Or, you can also have some DIY shading made. It even can come from some excellent varieties of water lily or other semi-aquatic floating plants and it would be more than enough.


Proper care for these living pieces of art come with a pleasant reward that lasts for years. As a symbol of peace, love, and friendship the time you will spend with them will be a beautiful, fulfilling part of your day.

So, if we take good care of them with every single detail in mind, they will take care of us too with genuine friendship and love in return till death do us apart.

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Pet Aquariums

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