Skip to Content

Adding Fertilizer to Aquaponics System: Should It Be Done?

Spread the Word!

Aquaponics may seem like a stand-alone system, the fish provides nutrients or fertilizer for the plants from their waste, so you don’t have to intervene. Aquaponics systems in fact don’t create the perfect environment for overall plant growth.

Additional nutrients such as Potassium (k) and Phosphorous (P) should be added to Aquaponics systems because they are not produced by the fishes in the tank. By testing the water and monitoring plant deficiencies, you would know which of these fertilizers needs to be added to the system.

In aquaponic systems, the waste from the fish is converted to nutrients for the plants. These Nutrients exist in the forms of Nitrogen such as Nitrates. Nitrates are responsible for plant growth and the production of flowers and leaves.

See our complete guide on aquaponics and what you need to know to run a successful system.

Plants as we know require Macronutrients such as Nitrogen, Phosphorus, and Potassium (NPK) as well as other micronutrients. So how do the plants get these other nutrients if it’s not produced by the fishes?

My Aquaponic setup

Build a successful system with these affordable equipment from amazon, that I’ve tried & tested on my setup.

Click on the links to see them

Best water pump
Best Water Pump
Best air pump
Best Air Pump
Best air stone
Best Air Stones
GROWNEER 25 Packs 4 Inch Garden Slotted netcups
Plant Net cups
Bio Balls
Bio Filter Medium
API Freshwater Master Test KIT 800-Test
Water Test Kit
The Aquaponic Equipment Essentials that I use in my system.

What Additional Nutrients is Required Aquaponics?

Adding Fertilizer to aquaponics systems requires monitoring the plants for deficiencies.

Also, the type of fishes in the system and the food you are feeding also plays a role in the nutrients that are available for the plants.

Aquaponic plants require a ratio of Nitrogen to Phosphorous to Potassium (N:P:K) and other trace nutrients such as boron, copper, chloride, zinc, magnesium, calcium, iron, and manganese. All these nutrients play an integral part in proper plant growth.

Luckily most trace nutrients are provided by the food which are being fed to the fishes. So take a look at the ingredients in your fish food and you will have an idea of what is also being provided for the plants.

So lets deal with the Macro or larger nutrients, (N,P,K)

We know that plants in the aquaponic system are already provided with copious amounts of Nitrogen from the breakdown of waste by the bacteria to produce Nitrates. The form of Nitrogen (N) that plants love!

But what if the system is low in Nitrogen?

Nitrogen deficiency symptoms?include poor plant?growth, and leaves become pale green or yellow because they are unable to make sufficient chlorophyll.

This can be easily corrected by adding more fish to the system.

Potassium deficiency symptoms in plants include brown scorching and curling of leaf tips as well as chlorosis (yellowing) between leaf veins.

Potassium can be added to aquaponics systems by either directly spraying potassium onto the leaves of the plant which has to be done periodically.

Another way to add potassium to the system is by adding over-ripened or dark to black banana peels directly into the biofilter. Kelp meal concentrate also does a good job. This would create a slow release of potassium into system and would prevent shocking the plants.

Phosphorous deficiency in plants will show stunted growth, darkening of the leaves near the base of the plant, and a purple or reddish color.

Phosphorous can be added to the system by using chlorine-free super triple phosphate or rock phosphate in a quantity of 20 to 40 ppm or parts per million in your system for fruiting plants.

I have made a table that would make it easy to identify the deficiencies and how to correct them.

NitrogenStunted Growth, Pale color, Light green yellowish leavesAdd more fishes to the system
PhosphorousStunted growth, darkening of the leavesAdd super triple phosphate or rock phosphate
PotassiumBrown scorching and curling of leaf tipsadd Kelp meal or old banana peels to biofilter
MagnesiumPaleness from leaf edges. Edges have cup shapes foldsUse leaf spray, such as Epsom salts
IronLeaves are pale, No spots, major veins GreenAdd Fe-DTPA soluble chelated Iron
CalciumPlant dark green, Drying starts from the tips, Tender leaves paleSpray calcium chloride onto leaves
Borondiscoloration of leaf buds. breaking and dropping of budsfound in trace amounts in fish food
CopperPale pink between the veins. wilt and Drop.found in trace amounts in fish food
ZincLeaves pale, dark spots on leaf edgesfound in trace amounts in fish food
Molybdenumspots on whole leaf except veins. Leaves light green or yellowfound in trace amounts in fish food
SulfurLeaves light green, veins pale greenfound in trace amounts in fish food
ManganeseLeaves pale in color, veins dark greenfound in trace amounts in fish food
Plant deficiency, Symptoms and Correction

What Affects the Availability of Nutrients

Aquaponics systems require routine water testing in order to maintain the right balance of nutrients available for optimal plant growth.

Water in aquaponic systems is tested for pH, electrical conductivity (EC), and temperature.

Temperature Control – Hight temperatures can cause the bacteria which convert the ammonia into Nitrates for plants. When the ammonia concentration builds it would cause the plants and fishes to eventually die.

pH control is also critical since it would affect the fish’s stress levels and can eventually cause them to die. It also affects how well the plants absorb the available nutrients.

Electronic conductivity (EC), reduces the quality of the crops. It gives an idea of how much available circulating nutrient ions are available for the plants.

These three method of testing is commonly used in aquaponic systems and give a fair indication of the water quality and the available nutrients in the water.


The nitrification process in an aquaponics system is pH lowering by nature, because of this you may need to add both hydrated lime (calcium) and potassium at the same time to both controls the pH and maintain the available potassium in the system.

The hydrated lime would ten to suck up the available potassium in the system, making it unavailable for the plants and hence causing a potassium deficiency. To counteract this you would have to add Potassium into the system.

What Fertilizers are safe to add?

From the table above we can derive the fertilizers or nutrients which are safe to add to aquaponic systems.

DeficiencySafe to Use Fertilizers in Aquaponics
NitrogenAdd more fish to the system
Phosphorus Add super triple phosphate or rock phosphate
PotassiumAdd Kelp meal or old banana peels to the biofilter
Magnesium Use leaf spray, such as Epsom salts
Iron Add Fe-DTPA soluble chelated Iron
CalciumSpray calcium chloride onto leaves
Safe To use Fertilizer in aquaponics system

How do you make aquaponic fertilizer?

Most fertilizers for aquaponics can be found at the plant shop but some can be made at home such as.

  • Potassium – use old banana peels or kelp
  • Magnesium – Use Epsom salt available at any pharmacy

Furthermore, aquaponics?can rely on?compost tea?for any nutrient supplementation. Compost tea can be made by using a porous bag, and placing organic materials such as banana peels, cucumber peels, carrot peels, and other chopped vegetables.

The bag is then placed into the Biofilter with adequate aeration. The aeration or air bubbles promote bacteria to thrive and break down the copped vegetables within the bag causing them to release helpful nutrients for plants to grow.

When to use fertilizers on plants?

Fertilizers should be added during early mornings between 6 to 8 am and late evenings between 5 and 6 pm.

This is because, during the heat of the day, plants are at their highest level of stress during the mid-day sun. Stress prevents the plants from absorbing the fertilizers through their leaves.

The ambient temperature is much cooler during early mornings and late evenings. The plants are more relaxed and are able to absorb the nutrients through their leaves.

Spread the Word!

Free Plant Care & Gardening Guides