Light, CO2, and Nutrients: Why Aquarium Plants Struggle to Grow

Light, CO2, and Nutrients Why Aquarium Plants Struggle to Growм

Every aquarium enthusiast, whether a beginner or a veteran, may run into issues when trying to grow aquarium plants, treat their diseases, etc. To find a way out of such situations, it is crucial to understand the fundamental basics of why our plants grow, how and which factors affect their growth in the first place.

Most of you have probably heard or know that aquarium plants require light, nutrients, and appropriate water parameters. However, this is far from the whole story. In order to successfully maintain plants in an aquarium, it is crucial to understand the interplay between light, CO2, nutrients, and water parameters.

In this article, I will try to provide a basic scheme and action plan based on the principles of aquarium plant maintenance to help correct the situation.


We all know, or at least heard, that our plants play an important role in maintaining biological balance in an aquarium by absorbing nitrates and phosphates, which can accumulate in the aquarium, reducing their concentration.

Plants absorb carbon dioxide and release oxygen during photosynthesis, promoting the normalization of the oxidation processes of fish waste and dead organic matter. Of course, there are also other advantages that plants bring to the aquarium, (such as aesthetic value, breeding ground, feeding ground, cover, etc.)

However, it should be understood that aquarium plants require attention, care, and proper maintenance to be an additional link in the chain of biological balance.


Photosynthesis is a vital biological process performed by photoautotrophs that converts light energy into the energy of chemical bonds found in organic substances using photosynthetic pigments such as chlorophyll in plants.

In modern plant physiology, photosynthesis is more commonly referred to as the photoautotrophic function, which involves a set of processes that include absorption, conversion, and utilization of quantum light energy in various endergonic reactions.

One such reaction involves the conversion of CO2 (carbon dioxide) into organic compounds. In simple terms, photosynthesis in plants involves the conversion of water (H2O) and carbon dioxide (CO2) into glucose (C6H12O6) under the influence of sunlight.

During the day, the formula for photosynthesis can be represented as follows:

6CO2 + 6H2O = C6H12O6 + 6O2

At night, the reverse process occurs. It means that glucose (C6H12O6) and oxygen (O2) are consumed to produce carbon dioxide (CO2) and water (H2O).

Essentially, cellular respiration is the opposite of photosynthesis, as it breaks down glucose molecules to release stored energy, whereas photosynthesis uses energy to synthesize glucose molecules.

C6H12O6 + 6O2 = 6CO2 + 6H2O

The main conclusion can be drawn that for healthy plant growth, the following are necessary:

  • First and foremost, proper levels of lighting.
  • Adequate concentration of CO2.
  • Then, macro and micronutrients.
  • Water parameters.

So, when plants in an aquarium are not growing well, even experienced aquarists can find it really challenging to pinpoint the exact cause of the imbalance. Obviously, it’s even harder for beginners.

Therefore, in my opinion, it is essential that we should start from the simplest potential causes before going on to more complex ones.

Step 1: Check Lighting Sufficiency

This is probably the number one reason why many aquarists fail to grow their plants. Without adequate light, plants will struggle to produce the energy they need to survive and grow, which can result in stunted growth, yellowing leaves, and ultimately, plant death.

Different types of plants require different levels and types of light (shade-loving or light-loving), so it’s important to provide the right amount of light for the plants in your tank.

The height of the tank must be taken into account as well.

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Step 2: Check CO2 Sufficiency

Secondly, we need to address the question of CO2 level. Is there enough of it?

The importance of CO2 in aquariums may seem insignificant, especially in low-tech tanks. However, the existence of low-tech tanks does not change the fact that plants require CO2 just as humans require food.

According to the studies, in the dry plant mass, carbon usually constitutes around 40-50% of the total weight. This is because plants are primarily composed of organic compounds such as carbohydrates, proteins, and fats, which all contain carbon atoms.

Carbon dioxide is a vital ingredient that significantly boosts the growth of aquatic plants. Therefore, if we want our plants to grow and be healthy we need to provide enough CO2. Basically, it should be our priority along with lighting.

Nothing surpasses CO2 in improving plant growth.

The recommended concentration range for CO2 in a planted tank is between 15 to 30 mg/l, with 30 mg/l serving as the upper limit for fish and shrimp.

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Step 3: Check the Ratio of Light to CO2

A healthy aquarium environment heavily depends on achieving the ideal balance between the amount of CO2 provided and the intensity of lighting.

It is absolutely vital to remember that simply boosting lighting intensity WILL NEVER make up for a lack of CO2 and can even cause an unwelcome algal bloom. On the other hand, insufficient lighting can make it worthless even with a saturated concentration of CO2. In the worst-case scenario, too much CO2 may suffocate everything in your tank.

In addition, we need to understand that some other factors may also affect the situation, such as the overall quantity of plants in the tank.

For example, let’s say that you have only a few plants in the tank. In this case, a sudden increase in lighting and/or CO2 may be too much for them. As a result, there will be excessive algae growth.

That is why gradual changes in lighting and CO2 levels are always recommended. This allows you to gradually achieve a true balance without getting algae everywhere.

Step 4: Check Your Photoperiod 

When it comes to providing the right lighting regime for an aquarium with plants, there are a few options.

Depending on the tank setup, the recommendation varies from 8 to 12 hours of light day. Ideally, it should even simulate sunrise, zenith, and sunset in a stepped way.

However, many aquarists believe that if their aquarium plants are not growing well, they can simply extend the photoperiod (the duration of the light day) to compensate for a lack of adequate lighting. However, this is a common misconception, and it is important to understand why.

Extending the photoperiod cannot compensate for a lack of adequate lighting!

Once again, these problems cannot be resolved by simply extending the photoperiod. In fact, it can even make them worse.

Extending the photoperiod will merely supply more of the same inadequate or bad quality light, which can hurt the plants even more if the light is already inadequate or of poor quality. Unfortunately, the results will be the same – algae everywhere.

Plants require a specific amount and quality of light to thrive, and it’s crucial to provide them with the appropriate lighting regime to ensure their growth and health.

Step 5: Imbalanced Fertilization 

Another common mistake especially among beginners is not taking the issue of micro and macronutrients for plant growth seriously, assuming that the plants will get them on their own (for example, from fish waste or food leftovers) in the tank.

This is incorrect in most cases.

An aquarium’s macro- and micronutrients must be carefully managed because either too little or too much might damage plants. Under-fertilization can result in plant starvation, while over-fertilization can result in algae blooms.

Nitrate (NO3), Phosphorus (P) need to be monitored and kept at a ratio 1:10. Iron, magnesium, zinc, and other micronutrient imbalances can also lead to problems in the tank. The good news though is that we can quickly fix it by water changes or by applying fertilizers.

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Step 5: Check Water Parameters 

Do you know that water parameters directly correlate to mineral content and even CO2 level? Obviously, these factors will play a huge role in plants’ growth.

  • Soft water is typically more acidic and has lower mineral content. While hard water is typically more alkaline and has a higher mineral content.
  • Soft and acidic water contains around 70% of the total carbon in the form of CO2 and 30% as bicarbonate (HCO3-). While hard water (GH) contains more bicarbonate.

On the one hand, we can conclude that many plants typically require a sufficient amount of micronutrients for growth, which means that hard water is more beneficial for them.

On the other hand, very hard water contains less CO2, which also limits plants’ growth. This is because in soft water with a low mineral content and a slightly acidic pH, there are fewer dissolved minerals to react with CO2, resulting in more free CO2 in the water. In contrast, in hard water with a high mineral content and a higher pH, more of the CO2 will react with dissolved minerals to form bicarbonate.

It is important to note pH, temperature, and the amount of dissolved minerals all affect the precise quantities of bicarbonate and CO2 in water. Species-specific requirements for aquatic plants will also vary and have an impact on plant development and health.

In Conclusion

Growing aquarium plants can be challenging. However, understanding the basic principles is crucial to achieve healthy plant growth.

For plants to thrive, proper levels of lighting, adequate concentration of CO2, macro, and micronutrients, and water parameters are necessary.

Given the fact that all of these principles are interrelated and dependent on each other, violating any of them will have a terrible result on plants’ growth. In other words, it is not possible to compensate for the lack of something by adding something else – it simply WILL NOT work!

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