Do Your Fish Need To See The Sun?

There’s one difference from wild fish and fish in a fish tank and it’s that the fish tank should not get sunlight while in the wild, the fish doesn’t have artificial light and only use sunlight. The same goes for plants. That made me think.

Do your fish tank need sunlight? It’s not recommended to have your fish tank near a window. Some kind of light is needed for the fish to find food and the plants to photosynthesize.

There are some benefits of having a fish tank near a window and some downsides too. We will go through what will happen if you put a fish tank near a window.

Light is necessary

Light is necessary for growth and the well-living for most species. It is necessary for a plant to live.

In the wild, we don’t have artificial light as we do have in our fish tanks. The light in the wild is the sun. It’s the main light source of everything living on this planet. Even for us humans, and even for fish and plants.

Even if that’s the case the recommendation is to put the fish tank in a place where the sun doesn’t reach it. That’s because a fish tank is a much smaller place than the ocean.

Your fish tank contains hundreds of liters (or gallons) of water, while the water in the ocean is a lot more. Because of the limited space in a fish tank, the fish count per liter is a lot higher than in the ocean. With a higher fish count per liter comes more waste.

There are beneficial bacteria breaking down the waste in a fish tank, but they also live in the ocean. There are also other species eating the waste sinking down to the bottom in the ocean which we don’t have in our fish tanks. For example clams.

With more waste comes more nutrition for the plants and that’s good for the plants in the fish tank. While that’s true the plants are only able to use the nutrient for up to 14 hours a day and after that, the plant “sleeps”. And what’s good for plants is good for algae since it’s also a plant.

Algae use the same mechanism, photosynthesis, as the plants do to be able to live. It’s a major difference with algae from the plants you love to look at and the fishes love to hide in. I stated above that the plants are not able to use photosynthesis for more than 14 hours a day. That’s not the case with algae.

Algae are able to photosynthesize a longer time and taking care of the nutrition in the tank when the plants are sleeping. Algae are also a fast-growing plant making it consume more nutrition if it’s available.

What will happen to your plants?

The inhabitant in a fish tank that needs light the most is the plants. They would benefit from the sunlight if it shines into the fish tank. The sunlight is very intense and the plants are able to use the light for photosynthesis.

The photosynthesis is when the plant is taking carbon dioxide and water, combine it with light and convert it to sugar and oxygen. The sugar is the food for the plant and the oxygen is the waste product for the plants.

The nutrition the plant is getting is coming from the fish in the tank.

Since the sunlight is so intense the plant could, in theory, grow a lot with the availability of enough carbon dioxide. The water is sufficient in a fish tank. The problem is that it’s not enough carbon dioxide for the plants to make use of the sunlight.

Instead, the algae are using it since it’s more effective at utilizing the photosynthesis and are able to grow a lot faster if the nutrition is available.

From the beginning, the plants in the water were not growing in tanks. It was (and is still) growing in the wild. On planet earth, we have different climates and your climate might not be the same climate the plant is adapted to live in. That includes the hours of light during the day.

Different plants need different amounts of light to be able to grow. Plants needing a low amount of hours are usually growing much slower than plants needing a lot of light.

With slow-growing plants, the sunlight provides a lot of energy for algae to grow. With fast-growing plants, the growth of the plants could outcompete the algae for the nutrition when provided sunlight.

What will happen to your fish?

Fish are not needing the light the same way as plants do. Fish use the light to be able to see, just like us! A fish needs to be able to see to be able to find food and to find a partner.

But, in the deep parts of the ocean, there are living fish too. These fishes don’t need the light as much as the fish living closer to the surface do.

There is only one kind of fish living in the dark conditions that we are able to buy for our fish tanks. It the blind cave fish. It has no eyes or pigment and is adapted to live in dark caves. It’s the only fish because it goes well with other fish in a fish tank.

Another reason is that the fishes living deep in the ocean are living under high pressure. When brought up near the surface, the pressure lowers and they die. Also, we tend to buy colorful fishes and the fish living deep in the ocean don’t need a lot of colors because of the lack of light coming down to the bottom.

Since most of the fish in our tanks are living near the surface, wouldn’t they benefit from living with sunlight?

The truth is that they don’t really care what kind of light they are provided. As long as they get their day and night cycle and are able to see the food, they are happy.

Benefits of sunlight

There are some benefits of having sunlight shining on your fish tank though!

Fish have shown to improve their coloration when exposed to the sunlight. It’s also beneficial for the fishes eating algae. Since the sunlight is stimulating algae growth there will be more algae. And more algae means more food for those fishes.

And there the benefits stop. There are more downsides to having a fish tank close to a window due to the sunlight then there are benefits. The benefits are easily countered by the lamps.

The lamps for your fish tank are made to have the benefits of the sunlight but you have more control over the lamps then you have over the sun. The lamps are made to help the plants grow. They are made for you to be able to see the fish clearly and to enhance the colors of the fish.

You turn on and off the light yourself, which is a lot harder with the sun. You don’t even have to turn it off or on yourself, you could make a timer, or somebody else, do it for you.

When getting lamps for your fish tank, take a look at what they are made to do. You often have space for at least two lights. One of the light should help the plants to grow to compensate for the sunlight. That is if you have plants. The other lamp should help you to see the fish since there is no point in having a fish tank if you are not able to look at your fishes!

Light is meaningful for a fish tank but it’s best to be able to control it. It’s not possible to control the sun but a lamp is possible to control. The lamps have the same benefits as the sun and the downsides of the sun removed. That’s why it’s not recommended to have your fish tank sun shining on it.


I have been a fish keeper a big part of my life. During the years I have gathered and searched a lot of information about fish keeping and here, on the website, is where I share that information to help you with your fish keeping. I want to provide the information that I didn't get when I had a question about my fish tank.

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