Can Hydroponic Nutrients Kill Fish? [Some of Them Do!]

Hydroponics nutrients are a staple for any hydroponic grower as it is needed for plant growth and survival. However, is the used nutrient-rich wastewater a hazard to aquatic life such as fish?

Hydroponic nutrients, in liquid or powdered form, will generally not kill fish. Potassium, phosphorus, iron, calcium, and magnesium are beneficial to both plant and aquatic life. Ammonia and urea are the hydroponics nutrients that in excess can kill both fish and plants.

Knowing which nutrients to introduce and which ones to avoid is key in your system. There are some which should be sought after and some which should be avoided.

Will Hydroponic Nutrient Kill Fish or Aquatic Life?

Hydroponic nutrients will not kill fish or other forms of aquatic life if not in excess.

As long as the nutrients are properly dissolved and diluted, it will not kill fish or any form of aquatic life. Potassium, phosphorus, iron, calcium, and magnesium are beneficial to both plant and aquatic life. An excess of ammonia (ammonium concentration above 0.2g/L) and urea can start causing problems to bad aquatic life.

Ammonia and urea are avoided in hydroponics and aquaponics because these become toxic because it will be absorbed by the plants rapidly. Unlike in soil, bacteria can efficiently convert these elements into nitrite ions which are safer to absorb by plants.

High amounts of ammonia and urea will cause disease and even death in fish when the ammonium levels reach 0.2 g/L to 2 g/L, depending on what a fish species can tolerate.

Can Hydroponics Kill Fish – Infographic

How To Properly Apply Beneficial Nutrients

Beneficial nutrients can be applied in a number of ways. They often come individually in powdered form and can be used to augment for the health and growth of plant and aquatic life.

Phosphorus, Potassium, Iron, Calcium, and Magnesium often come in water-soluble powders. To properly apply these nutrients in a system, add an adequate amount of potassium powder (based on system size and type) on the growing medium and run water over the nutrient powder to dissolve it. The dissolved nutrients will be absorbed by the plant and the run-off will be caught by the reservoir.


Phosphorus (P) is incredibly important for plant growth. It is involved in photosynthesis, nutrient movement, genetic transfer, and transformation of sugars, among others. For aquatic life, deficiency may lead to poor growth and bone mineralization.


Potassium (K) is an important element in fish growth because it allows for vigorous levels of growth as proven by different scientific studies. Likewise, potassium is a third element in the all important NPK macronutrient trio (nitrogen, phosphorus, and potassium) for plantlife. Potassium is necessary for protein building in both plants and animals.


Iron (Fe) is another vital nutrient for both plant and aquatic life. In plants, it is necessary for photosynthesis. In animals, it plays an important role in oxygen transportation. It is one of the most important inorganic substances for life.


Calcium (Ca) is a secondary nutrient for plants for the formation and stability of cell membranes. For aquatic life, it is necessary for the formation of strong skeletal structures.


Magnesium, in plant life, is the central core of chlorophyll molecules which is a necessary molecule in photosynthesis. Deficiency will lead to stunted plant growth. In aquatic life, an adequate amount of magnesium is for good weight gain.

How about Hydroponic Nutrient Solutions

Likewise, commercially available premixed hydroponic nutrient solutions, dry or liquid, can be used and it will not kill any aquatic life. It can be applied to the reservoir or the grow medium.

Be careful reading the chemical composition of the nutrients. Ammonia and urea should be avoided because an excess of either is poisonous to plant and aquatic life.

If you are unsure of the composition of the nutrients you are using, it may be necessary to dilute it by adding equal parts tap water.

Why Nutrients and Fish Are Relevant to Hydroponics?

Hydroponics is about efficacy and efficiency. Efficacy in the sense that optimal plant growth is achieved by controlling growing conditions which include the light, temperature, micro and macro nutrient uptake, acidity, and electrical conductivity, among others. Efficiency in the sense that everything is used as best as they can without any wastage.

Aquaponics is such a combination and a specific breed of hydroponics wherein wastage from the fish in the reservoir is used as nutrients for the plants in the grow bed whereas the plants filter and clean the same waste-filled reservoir back to the fish in the reservoir.

Though a symbiotic setup, aquaponics is by no means perfect since not all the nutrients available in the fish wastage are enough to sustain optimal plant growth so it will be necessary to manually add specific nutrients.

How to Dilute Hydroponic Nutrients in Wastewater

Used hydroponic or aquaponic wastewater is still nutrient rich and is beneficial for both plant and animal growth. The wastewater can be used on aquariums, indoor pots, or garden beds.

To dilute hydroponic wastewater, it is necessary to add an equal amount of tap water to the solution. By doing this, the nutrient concentration is so further diluted as to ensure the solution is safe for both plant and aquatic life.

What Are The Optimal Nutrient Levels

The optimal nutrient levels for your system will vary because of the different factors at play. For example the size of the system, the plants being grown, and the aquatic life present, among others, play an interconnected role.

The optimal nutrient levels can be gauged by research institutions for an appropriate fee. All they require is a sample of the reservoir water. Institutions like Hortus Analytical provide analysis services to agricultural enterprises, analyzing soil and the nutrient content of a hydroponics reservoir. Results provide the current nutrient level and composition, and the corresponding optimal levels for each.

Foliar Spray Alternative

Another way to add necessary macro and micronutrients to your system without necessarily applying them to the system directly. Foliar sprays operate by dissolving nutrients and spraying them directly on plants’ leaves. The stomata is an entryway not only for the exchange of gases but also nutrient salts.

Studies have shown that foliar sprays have proven effective in inducing plant growth and development. It increases yield, alleviates the adverse effects of salinity, and increases vegetative growth.

The best part is that this is extremely simple to operate. Simply dissolve the nutrients in a bottle and use a sprayer to apply nutrient-rich mist on the plants. The principle could also apply to organic fertilizers.


  1. Hydroponic nutrients in either dry or liquid form will not kill fish if properly dissolved and diluted.
  2. Many nutrients beneficial to aquatic life are also beneficial to plant life. Nutrients not recommended are ammonia and urea because they can be deadly to both aquatic and plant life.
  3. Micro and macro nutrients can be applied through foliar sprays which has proven effective in inducing plant growth and development. is part of the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites like mine to earn advertising fees by promoting good quality products. I may receive a small commission when you buy through links on my website.


Similar Posts