How Do Plants Get Nutrients in Aquaponics.

Students looking over lettuce in an Aquaponics system

Students looking over lettuce in an Aquaponics system

The primary source of plant nutrient in an Aquaponics system is the fish food.  The beautiful thing is that the fish food is a  one source of nutrient for the plants and the fish, a very important principle in working towards sustainability.|

Two uses from the same resource.

It is therefore important to choose a good quality fish food to feed your fish.   There is no magic happening in the body of the fish whereby it can mysteriously produce a full range of plant nutrient from a poor input for it's own needs. It is basic logic that a good quality, balanced fish food pellet will serve the overall Aquaponics system very well.

However, there are three elements that do not come into the Aquaponics easily from the fish food input.
1 Potassium, 2 Calcium, 3 Iron.

Happily, we can supply the Potassium and Calcium to the system when adjusting the system pH upwards.  As you would know, the natural state of affairs in a well found Aquaponics system is that the pH is always slowly drifting downwards, so there is a need to adjust the system pH upwards periodically as required.

Iron is added in the form of Iron chelates as required when the plants exhibit some iron deficiency or on a regular basis , say once every three months.

A good way to provide all the micro nutrients the plants require is to make sure you are running your system with the incorporation of some media beds.  Media beds provide a wonderful environment for the development of what I like to term, a “Nutrient Bank”. Over time we find that there is a build up of fine solids in the media beds, we find that worms take up residence, or we add them. The worms do what worms do to all organic material. They move about in the media bed and process the solids collected there. The solids are reduced in volume by up to 80% by this process and locked up nutrients are released.

Additionally, and very importantly we make good use of our own home-grown compost teas. Every Aquaponics gardener should become a master composter.  The compost tea so produced provides a myriad of plant nutrients for the system.   Compost tea has other important uses in our Aquaponic garden…..but that is the subject of another post.

Find out more about these very important info pieces by attending one of our training programs.

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5 thoughts on “How Do Plants Get Nutrients in Aquaponics.

  1. Hi Murray,
    We built up our system 3 weeks ago. From the beginning our PH was very high, 8.0 and is slowly going up. We discovered it can vary +/-0.1PH depending on the temperature of the water. We have no heater so the water can vary from 58F to 68F in a day.

    We cycled it for one week with no fish. The next week we bought I think 50 baby gold fish to try to establish the cycle (and to get practice). For the last 2 weeks there have been fish in the system, but we are finding 1 or 2 dead a week.

    Your article says we should expect PH to go down. But it doesnt seem to be doing so. We have a few plants in there just to help with establishing things but they are not doing too good.

    The water is municipal tap water. I’m wondering if the municipal tap is too hard, I read from some aquarium forums that CaO3 is the cause and wont go down on its own because of it. Can you give us some insight on this? Can we expect the PH to fall? At what point?

    • Hi Andre,
      The system needs to finish cycling before you can expect to see some pH
      stability. It is not uncommon to see pH swings, sometimes large swings
      while the beneficial bacteria is establishing. Your fish may be dying
      because of the chlorine in the municipal water and this may also be
      inhibiting the progress of the cycling process.
      pH will slowly drop but not until the beneficial bacteria has become
      established in the system. This process is a direct result of the action
      and activities of the beneficial bacteria in the system, so it cannot
      become a factor until the cycling process is complete.

      Your plants are not doing so good because, if the cycling process is not
      complete there is little or no nutrient in the system for the plants to
      use. In any case, the build up of plant nutrient takes time. It is not
      uncommon for a system to not reach its full bloom for 6 to 9 months. This
      depends a lot on the climate. Things happen more quickly during warmer

      I hope this helps.


    • Hi Andre,
      Things take time. Natural processes take time to establish. Measure the pH at the same time each day and minor fluctuations are not a problem. The pH dropping also takes time. Nutrient build up in the system also happens over time. The wonderful thing about AP is that it is an ECO system. This is also it’s difficulty in that modern man wants instant results.
      Are you using something like Maxicrop to help with the plant nutrition in the early days of the system?

      The municipal water may well be very hard in which case the system may never fall. The process that would normally be used to lower the pH is used up dealing with the CaO3 in the town water. The town water may also have chlorine or worse still chloramine. This may be causing difficulties with your fish. You may need to treat your top-up water in a separate tank before adding it to the system. Bring the pH down by adding hydrochloric acid to the water in the separate tank. Hydrochloric acid will use up the CaO3 and leave behind some salt. There will be very minor salt in the top-up water so it is not a concern.
      I hope this helps.

    • I would be hesitant to add HCL after I lost 90 percent of my fish after adding it. I too have the same problem with high PH (over 8). I bought a test kit that measures water hardness and found that Magnesium and calcium were high. Once the system cycled for quite a while the PH came down to around 7.5. There are probably carbonates in the water as well that make PH control difficult. But once those carbonates break down your system PH could crash so keep an eye on it. One thing I discovered that Mozambique Tilapia can handle water temps as low as 47F with a higher PH, but you lower it, and they all die.

      • Thanks Bill,
        I guess I am fortunate that I am able to use input water that is very close to 7.0 pH. I have many clients that have experienced you situation. They pre treat their input water in order to have a consistent control of pH in the system. Interesting comment re the Tilapia Temperature tolerance.