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Fertilization of Planted Aquariums


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Hello everyone, today we will discuss the functioning of aquarium fertilizers, the different types of fertilizers, their uses, how to choose them, how to dose them, control their presence, frequently asked questions, and my personal remarks (each example is indicative and not intended to promote one brand over another).

These pieces of information are the result of my own inquiries and research on various websites and contacting production/sales companies. Therefore, feel free to share your opinions and experiences to improve this content as much as possible.

Why use fertilizer?

Fertilizers are organic or mineral substances, often used in combination, that are intended to provide plants with additional nutrients to improve their growth, increase yield, and enhance crop quality. The act of applying fertilizer is called fertilization.

Fertilizers have been used since ancient times, where bone phosphates, calcined or not, animal and human manures, and ashes were added to the soil to provide phosphorus, nitrogen, and potassium.

Plants deplete the reserves of nutrients from water and soil, which leads to a decrease in nutrient levels in our aquariums. That’s why it is recommended to fertilize them in order to replenish these substances and ensure healthy plant growth while preventing deficiencies.

Each fertilizer has its own characteristics, and each composition is different, making it adaptable to the specific needs of your plants.

Types of Fertilizers

There are two types of fertilizers for our aquariums:

1. Complete Fertilizers: These fertilizers are rich in trace elements and provide a wide range of minerals, allowing for a comprehensive and cost-effective fertilization with a single product. However, one drawback is that they may add a surplus of certain nutrients that may not be lacking in our aquariums, which necessitates regular water changes to avoid excessive levels of certain risky nutrients such as copper (Cu). Complete fertilizers are available in two ranges:

  • Complete fertilizers with nitrogen (N) and phosphate (PO4) for well-planted aquariums with fast-growing plants and/or low population density (e.g., Tropica Specialised Nutrition).
  • Complete fertilizers without nitrogen (N) and phosphate (PO4) for less planted aquariums with medium to slow-growing plants and/or higher population density (e.g., Tropica Premium Nutrition).

2. Specific Fertilizers: These fertilizers are rich in a particular element such as iron (Fe) (e.g., Seachem Flourish Iron) or potassium (K) (e.g., Easy-Life Kalium). They allow precise control over the levels of each nutrient, which enables customization of the parameters according to specific requirements. However, specific fertilizers can be expensive to purchase despite their long-lasting nature, and they require a good understanding of plant needs, nutrient consumption, and concepts such as the law of the minimum. There are as many specific fertilizers available as there are modifiable parameters in our aquariums and even more.

How to Choose the Right Fertilizer?

The first question to ask is, “What are my needs?” Before choosing the products to add, it is essential to define your specific needs as each aquarium is different. To do so, follow these four important steps:

  1. Identify the plants present in your aquarium, their needs, and their growth rates.
  2. Assess the lighting conditions; intense lighting increases nutrient consumption by plants, while weak lighting reduces their nutrient requirements.
  3. Determine the CO2 level in your aquarium; higher CO2 levels lead to increased nutrient uptake by plants.
  4. Perform multiple water tests to determine the nutrient levels and identify possible deficiencies. Note that iron tests may only detect one type of iron, even though there are multiple forms available (a JBL test kit will detect iron present in JBL fertilizers but not necessarily in others).

In conclusion, consider these four steps and set a budget to choose the most suitable fertilization method for your needs.

Fertilization of Planted Aquariums

How Much Fertilizer Should I Use?

Each person may have their own method, but I prefer to calculate the number of milligrams injected per milliliter of product to achieve the most accurate value. To do this, start by conducting tests to determine your baseline values (TDS, iron, potassium, phosphate, nitrate, etc.) and then calculate the amount of product you need to reach your desired value.

(Tank volume in liters) x (mg/L desired) = Total desired mg

Total product mg / Total product ml = Concentration of the product in mg/ml

Total desired mg / Concentration = Number of ml of product needed

For example, in my 100-liter aquarium, if the iron level is 0 mg/L and I want to achieve a level of 0.2 mg/L:

100 x 0.2 mg = 20 mg

I have a product called “Seachem Flourish Iron” with a label stating it contains 10,000 mg/L of iron: 10,000 mg / 1,000 ml = 10 mg/ml

20 mg / 10 mg = 2 ml of product needed


Each fertilizer has its own specificities, advantages, and drawbacks. It is best to determine your needs to choose products that will be useful to you. I recommend using tests from the same brand as your fertilizers, as this generally facilitates the interpretation of values. Ideally, stick to a single product range to avoid accumulating excessive trace elements between different products. Along with regular maintenance and testing, these practices will help you achieve the aquarium of your dreams.

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- Mahatma Gandhi

“The greatness of a nation and its moral progress can be judged by the way its animals are treated.”

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