What fertilizer can be used in your system without adversely affecting the health of the fish? It is important to understand the nutritional requirements of your plants, and you should also be aware of what nutrients are added into the Aquaponics cycle through the fish food. Essentially, only the nutrients and minerals that are introduced into the system can end up in the food. It is a closed loop system, if you don't somehow add it, it is not going to be available for the plants.

This page in the "aquaponics resources" section of the site deals with getting the nutrients and minerals to the plants in a fashion that won't kill the fish. This is not as "straightforward" as it might seem. Unfortunately, most common commercial fertilizers are harmful for aquatic life. They can be used in regular gardening, and in hydroponics, but not in aquaponics. Because water is pumped back and forth between the fish tank and the plant beds, it is simply impossible to add a fertilizer to the plants without also adding it to the tank. Even "solid" fertilizers have to dissolve in order for the plants to be able to take in the nutrients (plants can only absorb water-soluble nutrients and minerals). Once dissolved, there is nothing stopping the waterflow from bringing the fertilizers into the fish tank.

Understanding fertilizer: the NPK labels

All fertilizers will have N-P-K levels printed on them. And these levels will be different for each type of fertilizer. Especially in an aquaponics system, you can't simply buy the one that is "designed for the type of plants you grow". Even if it is safe for the fish, the "tomato plant food" may not be the right one for growing tomatos. Have I confused you? Maybe that is a good thing, let's try to un-confuse and explain this N-P-K thing. NPK defines the level (amount) of different nutrients that are included.
  • N stands for nitrogen, which is needed for a strong stem and good foliage.
  • P stands for phosphorus, which is beneficial to the plant roots as well asl flower and seed production.
  • K stands for potassium, which improves overall health and disease resistance.
In an aquaponics system, the Nitrogen is not a problem, this is produced by the fish (once the bacteria have processed the Ammonia fish waste into Nitrates). Generally, you will not want to add any fertilizer that contains Nitrogen. And in most fertilizers, Nitrogen is actually the highest number in the NPK-rating. The Phosphorus and Potassium however are not naturally provided and need to be added, as do the trace minerals such as Copper, Iron, Zinc, Magnesium, etc.

The "perfect" aquaponics fertilizer compound

The Mono-Potassium Phosphate fertilizer contains absolutely 0 Nitrogen and high levels of phosphorous and potassium. This is a good basis for building your own aquaponics fertilizer. Look at what your plants need to grow, and supplement when needed with fertilizers such as Potassium Chloride and Potassium Sulfate. These fertilizers allow you to increase the potassium levels without affecting the Nitrogen or Potassium levels. I have found these fertilizers to be available online through hydro-gardens.com (and I am sure many others).

Trace minerals in Aquaponics

There are a lot of other items that you should include, such as Iron, Boron, Magnesium, etc. I will add information on these trace minerals at a later date. Stay tuned!

Once more - remember the fish

I cannot stress this enough. Do proper research before adding anything to your system. Make absolutely certain that the fish can survive the fertilizer you add to the system. When in doubt, get another fertilizer. And, always isolate a few fish in a seperate tank and dissolve a little of the fertilizer into the tank (don't give the fish a mouthful, either). Watch the fish closely to see how they react for one or two weeks. Only when they are happy, return the fish to the main tank and add the fertilizer to the plant beds. Like I said, I cannot stress this enough. Don't take shortcuts here.

Youtube explanation on mixing hydroponic fertilizer

The following video is about hydroponics (growing plants with no soil, bit without the fish).