Do Aquaponics Systems Need Fertilizer?

Aquaponics systems imitate the natural balance of natural nutrients, meaning they do not use fertilizer to grow plants and vegetables. This is because in an aquaponics system, the fish waste in the water is converted by the beneficial bacteria into nutrients for the plants. This concept has many skeptics raised and pointed out the nutrient deficiency of plants grown in aquaponics systems, which leaves to the question: do aquaponics systems need fertilizer?

The answer to this question depends on how aquaponics grower manages their system. 

In a standard aquaponics setup, plants rely on the nutrients from the fish waste to grow and thrive in a soilless environment. So technically, aquaponics growers do not use any fertilizer to grow plants. But if the system becomes unbalanced because the fish waste does not have enough nutrients for the plants, a nutrient deficiency occurs, and the plants become nutrient deficient. In this scenario, an aquaponics grower may need to supplement organic fertilizers into their system to solve the nutrient deficiency problem in their aquaponics plants. 

This article discusses how aquaponics works and converting fish waste into plant nutrients. We will also address the question of whether an aquaponics system needs fertilizer?


The Aquaponics Cycle

How Aquaponics Works

In aquaponics, the plants are grown in the grow bed, and fish are placed in the fish tank. The nutrient-rich water from the tank that contains fish waste is fed to the grow bed, where billions of naturally occurring beneficial bacteria break the ammonia down into nitrites and then into nitrates. 

Plants absorb these nitrates, and other nutrients to help them grow. The plant's roots then clean and filter the water before it flows back into the tank for the fish to live. The clean and oxygenated water recirculates back to the tank, where the cycle will begin again.

How Does Fish Waste Help Aquaponics Plants Grow?

The main input of an aquaponics system is fish food. Fish eat fish food and produce waste in the form of ammonia, a compound that is usually toxic to both aquatic creatures and plants. However, through the nitrogen cycle, the ammonia is consumed by the nitrifying bacteria in the grow bed, grow media, walls of the fish tank, pipes, and other surface areas and turns them into nitrite nitrates that serve as plant fertilizer. 

You have three main elements to keep alive in aquaponics: the fish, plants, and bacteria. These three elements rely on each other to live.

The fish produce the waste for the bacteria to consume and convert into plant nutrients. Through the help of bacteria, large amounts of waste are converted into valuable nutrients. In return, the plants clean the water for the fish to live. 

Unlike other chemical fertilizers that pollute our environment and deplete the minerals in our soil, the fertilizers that come from fish waste are organic and do not have an adverse effect on the environment. 


Nutrient Deficiency in Tomato Plants

How Does Nutrient Deficiency Occur in an Aquaponic System?

Plant nutrient deficiencies can occur in aquaponics systems for several reasons, these are:

1. Fish Feed:

One of the most common reasons is simply that the plants are not getting enough of the nutrients they need. This can happen if the fish feed does not fulfill the plant's nutritional requirements.

Therefore, it is essential to fish feed with the best fish food in appropriate amounts to have a balanced and successful aquaponics system. To learn more about fish feed, read our article guide to fish feeding in aquaponics

2. Water Quality

Water is the medium through which plants and fish receive all the essential nutrients and oxygen requirements. Poor water quality will affect not only the fish but also the plants in aquaponics systems.

Monitoring the water quality will help ensure your system is balanced and running smoothly. The parameters you need to maintain in their ideal range are pH, temperature, dissolved oxygen, ammonia, and nitrate levels. 

3. Dissolved Oxygen

Plant roots need oxygen for respiration, a critical plant process that releases energy for root growth and nutrient uptake. During respiration, plants absorb oxygen through stems and leaves and roots.

Most plants need high DO levels, which may exceed >3 mg/liter of dissolved oxygen. Healthy roots with enough oxygen supply can absorb the required nutrients from the water and ensure proper plant growth. 


Aquaponics Fish

Nutrient Supplementation Through Organic Fertilizers

In soil gardening, plants get the nutrients they need to grow from the soil they are grown. Farmers resort to artificial chemical fertilizer, pesticides, and other chemicals in poor soil conditions to get the most out of their crops.

Aquaponics imitates the natural balance of natural nutrients and does not need to use fertilizer to grow healthy plants. In an aquaponics system, plants are fertilized through the fish waste found in the water, and, in return, the plants keep the fish healthy by filtering the water before it goes back into the tank.

However, sometimes a nutrient deficiency happens, and additional nutrients are needed to supplement the plant's needs. This happens when the fish feed does not contain all the nutrients needed by the plants or the fish-to-plant ratio is not implemented. You can monitor these nutrient levels through regular water testing and monitoring your plant's health.

Only natural or organic fertilizers should be added to aquaponics systems. Keep in mind that you have the fish to think of, and adding chemical fertilizers might affect the fish and other aquatic animals negatively. Below are the ‌nutrients for aquaponics plants and fertilizers that could address the deficiency of these nutrients.

Phosphorus (P)

Phosphorus is essential for photosynthesis and the formation of oils and sugars and encourages germination and root development in seedlings.

Signs of Phosphorus Deficiency:

  • Stunted growth of plants.
  • Darkening of the leaves near the plants' base.
  • Purple or reddish color of the leaves.
  • Spare leaf growth.
  • Low root development.

How to Supplement Phosphorus Deficiency:

The most common method to add phosphorus to aquaponics is rock phosphate. The supplement can be added directly to your grow beds. Avoid adding it directly to the water and your grow bed should be shaded from the direct sunlight to ensure it doesn't dissolve before the plant can absorb it. 

The best way to ensure that plants will not suffer nutrient deficiencies in aquaponics is to maintain the 6-7 pH water level, feed the fish with a balanced and complete diet and you can also add worms in your grow media bed to help break the solids and provide more nutrients for your plants. 

Potassium (K)

Potassium functions as a cell signaling via controlled ion flow through membranes controlling the stomatic opening and is involved in flower and fruit sets. Potassium is involved in producing and transporting sugars, water uptake, disease resistance, and fruit ripening.

Potassium deficiency in aquaponic plants needs treatment. Because a lack of potassium can negatively affect photosynthesis, which can also affect plant growth, it can also make the plant susceptible to infection or infestation that could lead to plant death.

Signs of Potassium Deficiency:

  • Older leaves of the plants show interveinal chlorosis and spots or scorching, which progresses to the younger leaves when the deficiency becomes more severe.

How to Supplement Potassium Deficiency:

  • By Spraying - You can use potassium chloride and spray it into the plant leaves. Repeat the process at least once weekly until you see your plants are healthy again.
  •  Add a potassium supplement to your fish food through kelp meal concentrate. Other options are adding potassium sulfate or potassium hydroxide to your fish food.

Iron (Fe)

Iron is essential for the proper photosynthesis of plants. It is used in chloroplasts and electron transport chains. Iron deficiency can be identified easily by using an iron checker that checks the iron levels in the water.

Another way to identify iron deficiency is checking if the plants' leaves turn yellow, but the veins remain green, called "choloris." Iron has to be added to the system as "chelated iron." The suggested addition is 5 ml per 1 m2 of grow bed. A large quantity of iron supplement will not harm the system but can cause discoloration of the tank and the pipes.

Signs of Iron Deficiency:

  • Yellowing color on plant leaves
  • Spots on immature leaves

How to Supplement Iron Deficiency:

To supplement iron deficiency in your aquaponic system, you need to add iron that the plants can absorb. This means using chelated iron. Adding chelated iron to your system will only be effective if your pH is 7.5 or lower. Your aim is 2 mg/liter, so you need to calculate your water tank's size and add the required amount of iron every 3 - 4 weeks. 

Calcium (Ca)

Calcium is essential for the plants' healthy growth and sturdy cell walls and helps maintain plants' strength and shape. It is involved in strengthening stems and contributes to producing flowers, fruits, and vegetables. Squash, tomatoes, peppers, and other fruiting plants are some plants susceptible to calcium deficiency.

It is essential to treat calcium deficiency to prevent ruining the harvestable fruits or vegetables. Low calcium levels can also stunt plant growth and can cause plant death.

Signs of Calcium Deficiency:

  • Black, dead areas of the young plant tissue known as necrosis.
  • Slight chlorosis to brown or black scorching on new leaf tips.
  • Fresh leaves are distorted with hooked tips and irregular shapes.
  • Stunted growth of aquaponics plants.

How to Supplement Calcium Deficiency:

  • One way to supplement calcium deficiency in aquaponics is to use hydrated (or agricultural) lime, which will supplement calcium and magnesium and raise the pH levels. 
  • Another way is by spraying calcium chloride mixed with some water on your plants. The ratio should be four teaspoons of calcium chloride per gallon of water. You can increase the dose if needed and spray once a week to your plants.


Plants need many nutrients for healthy growth. Aquaponic growers must be vigilant to prevent and treat nutrient deficiencies when it happens. Fertilizers can be used if your system is imbalanced or lack nutrients. In using fertilizers, keep in mind to always use fertilizers recommended for aquaponics systems to keep your fish healthy.

Thank you for reading. If you have any comments, feel free to leave them below! 

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