9 Signs Of Too Much Light On Aquarium Plants (& What To Do)

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Last Updated on 2023-08-30

Aquariums need just the right amount of light so that the various fish and aquatic plants can grow and thrive. One mistake beginners or those with less experience with aquariums make is providing too much light to aquarium plants.

Signs of too much light on aquarium plants include rapid algae growth, the leaves of plants turning brown or yellow, stunted plant growth, shriveled and rotting leaves, and the appearance of brown spots on leaves.

Read on to discover more about exposing aquarium plants to too much light. Learn about the signs you can watch out for and how you can regulate aquarium light. Happy reading!

Can You Give Your Aquarium Too Much Light?

Maintaining the correct level of lighting in your aquarium is of paramount importance for the well-being of fish and aquatic plants. Just as low levels of light can be harmful to the fish and plants for a multitude of reasons, giving too much light to your aquarium can be equally harmful.

So, yes. You can give your aquarium too much light, which can be bad. Too much light can make the fish stressed and can additionally cause rapid algae growth, higher water temperatures, and the deaths of certain species of aquatic plants. In the next section, we’ll explore these effects in more detail.

An important thing to keep in mind is the intensity and power of light in your aquarium. The recommended power is three watts per gallon of water on average.

What Happens If Aquarium Plants Get Too Much Light?

Too much or too little light isn’t good for your aquarium plants. Here, we list down 5 signs that indicate your aquarium plants are getting too much light.

1. Rapid Plant Growth

One of the very first signs you might see initially if there’s too much light in your aquarium is rapid plant growth. Plants need nutrients, carbon dioxide, and light to grow. If the required nutrients and CO2 are available and there’s plenty of light, the plants will grow rapidly. This is especially true for those species of aquatic plants that need – and thrive on high levels of light.

2. Increased Algae Growth

A very common indicator of high light levels in your aquarium is increased algae growth. This usually happens when, in addition to too much light, the water also has lots of nutrients available. These conditions are quite favorable to the rapid growth of green algae. The increased algae growth can become a problem for other aquatic plants. If it becomes too dense, it can block off the light and cause the other plants to eventually die.

3. Higher Water Temperature

Interestingly, high levels of light in your aquarium may increase the water temperature in some cases. And a slight increase in the temperature can be dangerous for some species of plants – perhaps even fatal. VHO-fluorescent and incandescent lighting are known to produce heat when they operate and can easily raise the temperature of the water in smaller aquariums if they’re turned on for extended periods.

4. Stunted Plant Growth

Stunted plant growth is another common problem caused by the availability of excess light in the aquarium. This happens when plants get too much light but not enough nutrients or carbon dioxide. Due to this, photosynthesis can not take place, which adversely affects the growth of these plants. In some cases, this may even prove to be fatal (more on that later).

5. Death of Plants

As shocking as it may sound, plants can die if there’s too much light in the aquarium. There can be many reasons behind this. First, as we mentioned before, excess light and not enough CO2 or nutrients can hamper the growth of plants. If this continues for some time, plants can eventually die.

Secondly, if algae in the aquarium get too dense and prevent light from reaching other aquatic plants, the latter can die. Furthermore, there are certain species of aquatic plants that simply can not thrive in high levels of light, such as the Java Fern. They prefer darker areas and may die if they are exposed to a lot of light.

How Long Should Light Be On In a Planted Aquarium?

Here’s an important question: how long should the lights be on in your planted aquarium? Ideally, you should keep the light on for about 9 to 12 hours a day. This duration is perfect. It ensures your plants get the light they need to thrive while keeping the harmful effects of having too much light at bay.

When deciding how long your aquarium lights should be on, you should also factor in the species of aquatic plants you have. Certain species, such as the Java Fern and the African Water Fern, need less light and can thrive on just 7 hours of it each day!

Signs Of Too Much Light In Aquarium Plants

Here are a few signs you need to watch out for that indicate too much light on your aquarium plants. In many of these cases, rapid and widespread algae growth due to more light brings about these problems.

1. Rapid Algae Growth

One of the biggest signs indicating excess light in the aquarium is rapid algae growth. As we explained previously, algae grow faster in the presence of more light and can spread throughout the tank. This growth is further accelerated if the water has an abundant supply of nutrients. Faster algae growth happens when the aquarium lighting is kept on for more than 12 hours. To tackle algae growth, the lighting time must be reduced to about 8 hours. More on that later.

2. Plant Leaves Turn Brown or Yellow

Aquatic plants’ leaves usually turn brown when photosynthesis can not take place. And one of the reasons for this is a lack of light. If your aquarium receives too much light, algae will grow quickly and block off the light to other aquatic plants. Hence photosynthesis won’t take place, and the plant’s leaves will start turning brown. Plant leaves can also turn yellow if there’s a sudden change in the lighting conditions and they receive less light.

3. Stunted Plant Growth

Another sign your aquarium is receiving too much light is stunted plant growth. This can happen for a number of reasons. Firstly, some species of plants do not need a lot of light and thrive in darker areas of the aquarium. If they receive plenty of light, their growth can be adversely affected. 

In another scenario, increased algae growth can prevent light from reaching plants. This can prevent photosynthesis from taking place, stunting plant growth.

4. Curling Leaves

If you notice your plants’ leaves are curling, heavy lighting conditions might be to blame. Again, this happens due to the spread of algae, which prevents light from reaching aquatic plants at the bottom of the aquarium. The leaves curl due to a lack of chlorophyll production. Adequate light is a key requirement for photosynthesis and chlorophyll production.

5. Rotting Leaves

Here’s another problem caused by the spread of algae due to the availability of excess light. The leaves of the aquatic plants might start to rot due to a lack of available light. Since the algae block off the light for these plants, the leaves will wither, rot, and eventually die. This might be especially true for more sensitive species that need plenty of light to grow and thrive.

6. Brown Spots on the Leaves

The appearance of brown spots on the leaves of aquatic plants can be caused by a number of different problems and conditions. In most cases, these spots appear due to a lack of light caused by algae. The brown spots for patches form in areas of the leaves that do not contain chlorophyll. So instead of being green, these areas turn brown. These brown spots and areas can grow in number and increase in size as the problem worsens. Eventually, the whole leaf can turn brown and die.

7. Plant Leaves Turn Purple or Red

Here’s another indicator to watch out for that signals an excess lighting problem in your aquarium: plant leaves turning purple or red. The leaves of aquatic plants turn purple or red when exposed to too much light can cause the chlorophyll present in the leaves to break down. Chlorophyll gives the leaves their green color. When it breaks down, the color changes to purple/red.

8. Leaves Shrivel Up

Here’s one more way aquatic plants can be affected due to excess lighting, even without the presence of algae. In certain sensitive plant species, the leaves get dried out and withered when they’re exposed to too much light. The leaves simply shrivel up and die because of the heat generated by the intense lighting.

9. The Plant and Its Leaves Show Signs of Bleaching

Bleaching in plants and leaves refers to the loss of their original, healthy color. Plants that are exposed to high levels of light often turn from green to a ghostly white color. Bleaching also occurs due to a sudden change in lighting conditions when the plant fails to adapt to the new conditions and can not absorb light.

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How To Regulate Aquarium Light

Knowing that you have an excess lighting problem is the first part. The next step is to fix that problem by regulating your aquarium light. 

Before you start implementing any of these tips, it is important to factor in the size of your aquarium, the species of aquatic plants you have, and the placement of the aquarium (if it has access to natural sunlight or other sources of light besides the aquarium’s own lighting). This will help you determine the best method of regulating your aquarium’s lighting.

Below, we go over some excellent tips and suggestions to help you regulate your aquarium’s light.

1. Determine What Kind of Lighting You Need

This is the important part. Before you go out and invest in lighting equipment, it is important to determine what kind of lighting and its power is best for your aquarium. As a rough approximation, aquariums need three watts of power to light a gallon of water on average. You also have to decide the color temperature that is best for the species of plants – and fish you have in your tank.

2. Consider Fluorescent or Incandescent Lighting

Fluorescent and incandescent lighting are some of the most popular types of lighting for aquariums. Although fluorescent lightbulbs are brighter than incandescent ones, they are more versatile, cheaper, and suit a larger variety of plant and fish species.

On the other hand, incandescent light bulbs and tubes are slightly more expensive and not as bright. They are better for smaller aquariums.

3. Consider LED Lighting

If you’re looking for other lighting types besides fluorescent and incandescent lightbulbs, you might want to consider LED lights. LED bulbs are more expensive but last longer and don’t produce heat. Do keep in mind that older lighting technologies can produce heat and raise the temperature of the water. LEDs are also fairly easy to set up.

4. Don’t Leave the Lights on 24/7

This is a really simple precaution you can take to prevent aquatic plants from suffering due to excess light. While it’s easy to implement, those who are new to fishkeeping and don’t have much experience with operating aquariums usually forget to turn off the aquarium lights. Remember, aquarium lighting should be kept on for only 10 to 12 hours a day, not 24/7. Not only is this better for plants, but it also saves electricity.

5. Get a Light Timer

A light timer is a device that helps you regulate and schedule aquarium lighting effectively. With a timer, you can set custom on and off times. This saves you the effort of trying to remember when to switch on and when to turn off the lighting. It is simply quite effective. If you want to buy a light timer for your aquarium, check out Amazon’s collection here.

6. Prevent Algae From Spreading

The rapid spread and growth of algae is perhaps the biggest problem stemming from heavy lighting conditions. It creates prevents light from reaching other aquatic plants and must be stopped. To stop algae from growing and spreading further, reduce the intensity of your aquarium lights and reduce the number of hours for which they’re turned on.

7. Use an Aquarium Lid/Cover

Using an aquarium lid or cover is a good way of preventing the aquarium from being exposed to unnecessary light. You can use a thin, opaque lid that can be mounted on the top for some time during the day.


Here, we answer other related questions you might have about aquarium plants being exposed to too much light.

Can Aquarium Plants Have Light 24/7?

No, aquarium plants shouldn’t be exposed to light constantly. As we’ve discussed before, keeping your aquarium lights on 24/7 can negatively impact plant growth and cause them to wither and die. Moreover, keeping the lights on 24/7 also encourages the growth of algae, which can prevent light from reaching the plants. Check out this guide that explains why aquarium plants can not have light 24/7 in more detail.

Can Too Much Light Kill Aquarium Plants?

Yes. Being exposed to too much light can easily kill aquatic plants. This can either happen due to the growth of algae, which can prevent light from reaching these plants. Or the plants can simply burn and die due to over-exposure to light. This article explains how too much light can kill aquarium plants.

What Is Considered Low Light For Aquarium Plants?

The average amount of light for an aquarium is three watts per gallon of water. Low light is light less than one watt per gallon of water. Most aquatic plant species can not thrive in very low levels of light, and those that can are quite tough. Such species include Java Fern, Amazon Sword, and Anubias. These plants need minimal light for a small period (about 7 to 8 hours). Here is a helpful guide that explains bright, medium, and low light conditions for aquatic plants.

What Is Considered Bright Light For Aquarium Plants?

Bright or heavy light is light greater than 5 or 6 watts per gallon of water. Bright light isn’t needed for most aquatic plant species to thrive and may bring about more harm than good. Bright light is more than enough for photosynthesis to take place and encourages the growth and spread of algae. Exposure of aquatic plants to bright light is comparable to exposure to direct sunlight. 

Final Thoughts

In this guide, we went over some signs that indicate an over-exposure of aquarium plants to light. We discussed the problems associated with heavy lighting and how it can wreak havoc on the fragile ecosystem in your aquarium. Additionally, we put forth some ways to regulate your aquarium’s lighting and prevent these problems from occurring. 

We hope you enjoyed reading this guide and found answers to all the questions you had about exposing aquarium plants to too much light. Thank you for reading!

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