Do Your Fish Tank Plants Really Need A Light?

When you look into a fish tank you most of the time see a light shining. At first, I thought it was to be able to see the fish. The plants are using the light while growing on land, that’s something I learned early in school. But underwater plants? Is the light coming down to them?

Do fish tank plants need a light? They need light, but not all plants actually need a lamp in the fish tank. Some are able to live on the ambient light from the room.

It seems like the plants in the water works like the plants on land. They use photosynthesis to get the energy necessary to grow. We will go deeper into how it works and what’s the difference in this article.

How plants work

Plants. They are living organisms just like you, me and every other organism on this planet. And every living organism requires some source of “fuel” to be able to live.

You and I are humans. Humans need food to be able to live. It’s necessary for all of our mechanisms in the body to be able to work. The same goes for plants. But plants don’t eat food, they produce it themselves. They utilize something called photosynthesis instead to provide them with energy to be able to grow.

We will go through, short, how the plants work, as a reminder from school.

The photosynthesis is a conversion of the light energy from the sun into chemical energy. The chemical energy is used by the plant to repair, grow and reproduce itself.

This conversion is happening in the leaves of the plants. In the leaves, there are organelles called chloroplasts. In the chloroplast is where the magic happens. But, photosynthesis is the conversion of light energy to chemical energy, how is the light coming into the chloroplast?

The light is absorbed into the chloroplast with the help of a molecule called chlorophyll. Now the light is absorbed, how is the plant converting it into chemical energy?

To convert the light into the energy the plant needs it uses carbon dioxide from its surroundings. The carbon dioxide is getting into the chloroplast through the leaves. With the carbon dioxide, the plant combines water and the light it has absorbed into sugar which is the energy source the plant uses. It produces a rest product for the plant too. It’s oxygen which is vital for other organisms to survive, but not for the plant.

The three vital components for a plant to survive are carbon dioxide, water, and light. The reaction looks like this:

 Carbon Dioxide + Water -> Sugar + Oxygen

It’s exactly the same reactions in land-growing plants and water-growing plants. But they live in different conditions.

Water-growing plants vs land-growing plants

The plants growing on land and the plants growing in water works the same but live in vastly different conditions.

We have already determined which three factors which is crucial for a plant to survive. All these three factors are available in both types of environments. They have different difficulties obtaining carbon dioxide, water, and light.

If we start with carbon dioxide, the land-growing plants have an easier time to obtain carbon dioxide than the plants growing underwater. The carbon dioxide is diffused much slower in the water and thus making the available carbon dioxide less under the water than in the air.

To combat the shortage of carbon dioxide under the water the submerged plants have developed leaves without a wax coating. Without the coating, it’s much easier for the leaves to absorb carbon dioxide.

A lot of plants submerged in the water have developed smaller leaves since they have an easier time to absorb the carbon dioxide than the larger leaves often found on land-growing plants.

With these little tricks, the plant is able to absorb the right amount of carbon dioxide even if there is less carbon dioxide to absorb.

The next challenge is the water for the plants. The plants submerged in water doesn’t have any problems with absorbing water though. They are submerged in it! This is the challenge for the land-growing plants and that’s why you need to water your potted plants. How silly would it be to water your aquatic plants!

The last challenge is for the plant to get enough light. Land-growing plants have the sun shining right on them giving them sufficient light. It’s much harder for a submerged plant to get a sufficient amount of light to be able to photosynthesize.

That’s due to the debris and minerals in the water coming from rocks and the waste from fish etc.

To combat that the chloroplast is situated near the surface of the leaf in the water-growing plants. That differs from the land-growing plants where the chloroplast is situated further into the leaf. This will make sure the submerged plant can use as much light that reaches the plant as possible.

The underwater plant has also adapted to be able to live on less light than the plants growing on the land, especially the plants growing deep in the ocean where very little sunlight is able to travel.

Floating plants are built the same way as the land-growing plants. They use carbon dioxide from the air and also the sunlight that hits the plant directly.

The plants that are growing underwater has adapted very well for the conditions they are living in.

The light for a fish tank

I have written an article where I go through the lighting in general both for your fish and for your plants. In that article, I go through the different kinds of light you could use for your fish tank with the benefits of each type of light.

For a plant in a fish tank, it’s best to have an LED light produced specifically for plants. That’s because with the technique we have today we are able to produce LED lights that have more light from the wavelengths stimulating photosynthesis. Those wavelengths are in the red and blue spectrum.

A lot of people think green is absorbed in larger quantity since the leaves on a plant is green. But it’s the complete opposite! What makes the leaves green is the light reflected from the leaves. It’s mainly the green wavelengths since you see the leaf as green. When the light is reflected it means it’s not absorbed and therefore not used in photosynthesis.

So, choosing a light for your fish tank should be easy. Just get a lamp made for plants and check it’s in the red and blue spectra. Preferably a LED light.

The light requirement for plants in a fish tank

The average light demand for a plant in a fish tank is about 20 lumen per liter of water (or 75,8 lumens per gallon). Some plants are able to grow with more and some are able to grow with less.

What decides is how they are adapted in nature. Some plants growing at deeper parts of the ocean are able to use the most of small amounts of light and are demanding less light than a plant growing close to the surface, getting more light.

A plant growing near the bottom is in the wild growing near the surface and are mainly needing a lot of light. That’s a little paradoxical in a fish tank since you most of the time have other plants too. The other plants are most of the time larger and are needing less light than the plants on the bottom.

That’s because the larger plants are typically growing at deeper waters in the wild. The larger plants will shadow the bottom growing plant in a fish tank and giving the smaller plant insufficient lighting. When having these small plants growing near the bottom, be sure to plant it som no other plants are blocking the light and that you have sufficient lumens for the plant.

Lumen is how much light a light source is giving. The lumen is always written on the package of the light. For the plants in a fish tank, it ranges from 10 lumens all the way up to 40+ lumens per liter.

To get better use of your light, be sure to use a reflector. The reflector reflects a lot of the light going upward down to the water. That makes more light reaching your light and your light will be better at using the light for photosynthesizing. It also makes it possible for you to have more light-demanding plants in your fish tank without the necessity to switch out the light source.

A source I use to look at how much light a plant is needing and much more information are Tropica is developing plants with different needs making you find just about every type of plant you want.

Another good thing with tropica is that you can get inspiration on how to plant that specific plant in your fish tank. They also grade the plants by difficulty depending on the light demand.

The risk of not having sufficient light

Some plants in a fish tank could live with just the ambient lighting in the room where the fish tank is situated. Especially if the light in the room is strong from for example the sun shining in, right onto the fish tank.

That’s not a good idea though since it’s hard to control the light from the sun. It’s easier to control the lighting with an automatic timer instead. Be sure to have enough light for your plants by reading about the light requirements for the different plants you are planning to have in your fish tank beforehand.

If you don’t provide your fish tank plants with enough light they are not able to photosynthesize and they will not get the energy they need to be able to grow and they will ultimately die.

Dying plants are going to give you bad water quality since the dead plants are being consumed by bacteria and starting to rot. The rotting process is releasing waste products into the water and it could be hazardous for your other living organisms in the fish tank water.

The water quality will be bad due to the waste products being produced and it will give you more work to manage the water quality and you run a higher risk of diseases killing your fish because of the quality of the water.

With dying plants could even fish, snails and shrimp start dying. Some fish, snails, and shrimps eat plants and when the plants disappear as a result of them dying. The food will be gone for these inhabitants. It’s probably not the main reason they will die because you will feed your fish tank providing them with food that way.

It’s more probable they die due to the lack of hiding places. Especially for the shrimps. Both fish and shrimps use the plants to hide in. If they don’t have anywhere to hide they are more exposed to predators. the exposure to predators makes the animal more stressed and I have concluded in this article, it’s the most common cause of your fish dying.

For fish tanks without filters, it’s crucial to have a working plant-life to provide the oxygen needed in the tank. The fish, shrimps, and snails need oxygen to live. With no air pump, the only way to get oxygen into the water is by the oxygen provided by photosynthesis from the plants. If the plants die, which they will with insufficient lighting, there will be no way of providing this type of tank with oxygen.

That will make your inhabitants die due to the lack of oxygen.

You will probably see more algae grow if you don’t provide your fish tank with the right amount of light. That’s because the algae are a kind of plant too but they are more efficient to use the light for their photosynthesis. If you have plants that die due to insufficient lighting, the algae will take care of the light provided and start to take over the tank.

I would rather look at a tank with plants then algae growing. Therefore I always make sure I have the right amount of light for my plants in my fish tank. That’s a good way to combat algae-growth due to the increased competition for the algae. With a fish tank with growing plants, the fish will thrive and the tank will get more life and look prettier.


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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