Understanding Iron in Aquaponics

Understanding Iron in Aquaponics
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Iron in Aquaponics *

*** Fair Warning: This post gets little technical! ***

UPDATE: See the bottom of this post to find out what chelated iron products you can use in your AP system!

Why is Iron Important? *

iron in aquaponicsBy mass, iron is the most plentiful element on the planet, and one of the oldest metals known to and used by humanity.  It is also an important plant and animal nutrient and thus, very crucial to your aquaponics system.

Iron is very reactive- that is, it exists in a variety of ionic states (from +6 to -2) but exists primarily as Iron++(II; Ferrous Iron) or Iron+++(III; Ferric Iron) and transitions readily between them depending on environmental variables.

For this reason, oxygen is an important component in many organic molecules that fix oxygen, or moderate REDOX reactions.

Animals & Iron

In animals, the most common iron containing substance is heme complexes, of which, hemoglobin is what we are most familiar with.  In hemoglobin, iron helps bind oxygen for transport throughout the body.

Plants & Iron

In plants, iron serves many functions but is an essential component in the production of chlorophyll, the site of photosynthesis.

Without enough iron, plants cannot produce enough chlorophyll, leading to retarded plant growth characterized by interveinal chlorosis. Iron is also a key component of cytochrome- a hemeprotein that plays a key role in ATP generation- the currency of cellular metabolism. 

In this capacity it is irreplaceable to both plants and animals.  Iron is also plays a major role in many other proteins and reactions.

Iron Availability (or Lack thereof) *

Unfortunately, because it is highly reactive, iron is typically unavailable.

It flits between soluble and insoluble forms, forms compounds with other minerals and in aerobic environments generally (as far as plants are concerned) plays hard to get.

The Issues with Iron in Aquaponics

This poses a problem for aquaponic producers.  Because systems are generally aerobic (and certainly aerobic in the root zone), iron deficiencies can often arise- even when there is technically plenty of (ferric) iron within the system.

In the aquaponic solution, iron is commonly available in one of two forms- reduced, soluble Ferrous Iron (2+) and insoluble, oxidized Ferric Iron (3+).

Ferrous iron is available to plants (soluble!).  Ferric iron is not (insoluble).

This is important to understand, because ferric iron is the more oxidized form, whereas ferrous iron is not.

In short, as soon as ferrous iron becomes soluble in aerobic envrionments it is often oxidized, becoming ferric iron or reacts with other compounds to become biologically unavailable (especially at high pH values when different hydroxides are formed).

Now, this relationship between oxygen and iron isn’t a full time thing.  In reality iron is flitting between ferrous and ferric states, but the dominant state in high pH and oxidized environments is ferric- and this means that your plants cannot take it up.

 iron in aquaponics

Why are these details important? *

Because they dictate how we examine the solutions.

Many practitioners throw rusty iron items into their systems falsely assuming that this will supplement system iron.

In a sense it does add to the reservoir of system iron, but not in a constructive or meaningful way.  All this does is introduce more ferric iron to the system- a form of iron that was most likely already in plentiful supply.

Other practitioners intentionally develop dedicated anaerobic zones, where ferric iron will be reduced by the oxygen free, anerobic environment to produce ferric iron.  This is a more compelling approach, especially in low pH systems, but still does not entirely address the problem of getting the reduced iron ion (Fe++) through the oxygenated aerobic zone surrounding the plant roots (especially in high pH systems where hydroxyl ions are plentiful!).

In low pH systems, ferrous iron has a much better chance of reaching the root zone, simply because there are fewer hydroxyl (OH-) groups to react with along the way, however even in the absence of hydroxyl groups, there are many other chemical obstacles to reaching the plant root zone in adequate quantities.

Plants have adapted to this issue *

This is a problem, but one that has not been overlooked by nature.

You see, plants have been contending for these nutrients for eons, and as a result have developed some amazing chemical means of hijacking ferric iron ions, tying them up, bundling them into the soluble, biochemical equivalent of the panel van, and delivering them, bound and gagged, to the root surface for plant use.

Plants also use a few other techniques to make iron available, including acidifying the root surface by excreting hydronium ions, and secreting iron reducing compounds.  But for the sake of aquaponic system management, this first biochemical iron fixing technique is what we will focus on.

Chelation – An Aquaponic Iron Fixing Technique *

This process is called chelation- that is, tying insoluble ferric iron ions and compounds to organic molecules to make them soluble.

Chelation is accomplished by special organic molecules called chelatins or chelating agents.  These are organic molecules that are specially designed to capture, or “dissolve” metals, of which iron is one.

In the plant world, chelatins are produced by the plant roots and leaked into the soil capture and deliver insoluble iron ions.

The most effective of these compounds are phytosiderophores which bind ferric iron very strongly, pulling them from the various insoluble precipitates and substances in which they most commonly occur.  These are special compounds produced by certain plants (phytosiderophores) and bacteria (siderophores) that are incredibly effective at binding iron.  The grasses (Poaceae), and especially barley are particularly effective at producing phytosiderophores for capturing iron.

(As a side note: a great deal of research is being done on using barley to produce siderophores for iron sequestration, and holds some interesting implications for aquaponic system where practitioners are willing to grow barley.)

Other Chelating Agents *

Other common chelating agents are amino acids, organic acids (especially humic acids), and polyphenols.

These are compounds that help keep the iron soluble and biologically available to the plants and algae in the system.  While these compounds can be introduced, and humic or “tea-water” solutions can be fostered and managed, they aren’t always enough to keep iron available to the plants- especially in systems with a pH or 7 or above.  In these systems, an artificial chelatin is often required.

Because I use peat potting mixes for all of my seedling germination and transplants, my systems typically maintain high levels of humic substances, however I still supplement chelated iron regularly.

Iron is one of the plant nutrients that must be supplemented in almost all aquaponic systems.

To supplement iron, chelated iron must be added to systems.

Admissible under USDA Organic standards, chelated iron is an artificially chelated iron ion- essentially, iron attached to an organic molecule to make it soluble.

By adding chelated iron, iron deficiencies in your plants can be avoided.

Forms of Chelated Iron *

The most common forms of chelated iron are:

FeEDTA: This is a slightly toxic form that aquaponic practitioners should not use.  This type of chelated iron is commonly used as an herbicide to kill broadleaf weeds.  It should not be used just  because of it’s toxicity, but also because it typically only effectively chelates iron up to the pH range of 6.3 or 6.4.  Above this range it is not a stable chelate.  So, using FeEDTA in your consistently pH 7.0 system represents a significant amount of money wasted in comparison to other forms of chelated iron.  For this reason I recommend that AP practioners do not use FeEDTA.  It is ironic that this is the most commonly sold and used form of chelated iron in aquaponic systems as it is fairly ineffective- the equivalent of modern “aquaponic snake oil.”

Fe DTPA:  This is what I recommend for most systems at pH values between 6 and 7.5.  It is commonly available at lawn and garden stores.

FeEDDHA:  This is what I recommend for systems above pH values up to 9.0 (let’s hope your pH never gets that high!), and the best all-round form of iron chelate- especially for starting systems.  Effective at a broad pH range, FeEDDHA maintains iron solublility in almost all of the water conditions encountered by startup aquaponic systems

Chelated iron fertilizer is available from many different suppliers.   I typically get mine from the local hardware store.

Common thinking about adding chelated iron *

There are two schools of thought on chelated iron addition.

Some say that chelated iron should be applied any time you see deficiency.  This is a reasonable and reactionary dosing method, but ultimately means that your plants must first suffer from iron depletion and deficiency before the problem is addressed.  In this scenario plant production can be negatively impacted.  The other (and better) school of thought is to apply iron at the standard UVI system rate of 2mg/L every three weeks.

Iron can also be applied through foliar application- using either chelated iron or ferrous sulfate mixed at low concentrations.  Foliar application is great for fast response, but because iron isn’t a mobile nutrient inside plant tissues, iron will have to be supplemented regularly using this method- a time consuming, and ultimately less effective iron supplementation method.

 chelated iron in aquaponics

Using this method, iron can be regularly dosed so that iron deficiencies do not arise in your system.

Cost of Chelated Iron *

cost of chelated ironWhile many practitioners complain about cost, when bought in the 5-10 pound bag, chelated iron is really not very expensive, and often even in large commercial systems, will last for many months.

At the dosing rate above, a 10 pound, $15 bag of chelated FeDTPA will last well over a year, or less than $1 per month.  At higher iron concentrations it will last much longer.

* The above material is copyright Nate Storey, Ph.D *

We hope that helps you better understand the iron in your aquaponics system. For more discussion, please visit our Google+ Community on Aquaponics.


We’ve had several folks ask about where they can get good iron supplements and how much they cost. Here is that info:

Chelated iron products you can find on the web – available on Amazon and other online stores:

“Miller DP”- DTPA (What we use- on the shelf or ordered through Ace hardware)
“Sequestrene”- DTPA (5 lbs bag on Amazon for $57)
“Miller FerriPlus”- EDDHA (SunshineGardensFl dot com; 1 lb for $20, or 20 lbs for $300)
“Sequestar Iron 6% Chelate”- EDDHA (RoseCare dot com; 5 lbs for $73)
All of these products will work great in your system!

Update #2: Beware red dye in Miller’s FeEDDHA *

A friend and blog reader let us know about this product that seemed to have turned his entire system water red after adding 3 ounces. It appears the Miller’s product contain some kind of red dye! Check to make sure you’re using good iron supplements without dyes!

Learn more. Watch the video:



  1. I have Thank you for making this topic so clear.
    I looked through Ace Hardware online but have not been able to locate this 10 pound, $15 bag of chelated FeDTPA product. Maybe it’s unique to your location. But if you have a link please include it.

    • My pleasure Bob. I’m glad you found it useful. Check out the list of other chelated iron products we recently added at the end of the post. They’re all available online, and good products to use in AP systems.

  2. Great info Thank you very much. Seems most products out there have traces of lead & arsenic in them. Is there an acceptable level in ppm for theses metals? Is there an EDDHA out there that doesn’t have these metals in them? Thanks again

    • Yes there are acceptable levels, and to be fair, many of the iron products out there don’t contain high levels of arsenic and lead, or even bioavailable arsenic and lead. But my concerns are that if pH changes, or other environmental variables change, it could become available in your system.

      If you use chelated iron (EDDHA, DTPA, EDTA), there is no lead or arsenic in these forms of iron. In fact, cheating agents can lower the amount of bioavailable Lear and arsenic in your system overall.

      • I bought Grow More EDDHA 6% chelated iron. No where on the label does it mention any other metals. But when I looked it up on Washington. State department of agriculture site it listed it as having 0.5 ppm arsenic &3.8 ppm lead & 0.05 ppm mercury. Millers also had these metals. My current ph is above 7 thats why I was looking for EEDHA. Any suggestions? Thanks

    • No, I haven’t. It looks like they’re using a natural chelating agent. It probably works fine, but I would check on the effective pH range before you invest.

  3. Nate I am having a problem with finding a EDDHA iron that does not turn my water red. Sprint 138 is red. Miller ferrisplus is red. No info on the labels about containing dye. Is they any EDDHA iron that you have tried and could recommend?

    • Hmmm. That’s interesting. I’ll hopefully be talking with Miller sometime soon about this. We’ve always used Miller FeDTPA- there are no dyes in that. I’m wondering if they’re putting dyes in all of the bulk commercial iron they produce, and folks on the internet are buying bulk and then breaking it down. I’ll look into this. . .

      • Her is the reply I got from the sprint138 FEEDDHA manufacturer.

        —-Thank you for your inquiry about Becker Underwood’s Sprint 138 EDDHA micronutrient iron.
        In reply to your question, the iron ingredient of the product is inherently red and there are no dye colorant additives.
        So, what you see in the water solution is the natural color of the iron content.


        Joe Lara
        Product Manager

        • Yeah my water looks like red Kool-aid

          • Bummer Rob. Sorry about that! Tell your fish not to drink it.

        • Interesting. This is news to me! I’ll check around some more to see if this is pretty standard. I’ve used nutrient mixes in the past with FeEDDHA and no coloring at all, so I’m not sure what to think.

  4. In regards to the red water issue, sequestar 6% FeEDDHA also turns the water red.

    • Could the change in color be caused by a chemical reaction between the FeEDDA and some salt that’s dissolved in the water?

  5. Hello,

    I have found chelated iron 13% in a specialize store (28$ for 1 kg), but i have no indication of what dose i need to put my system (mgL)

    My system is approximitely 500 L

    Thank you!

  6. I got GrowMore EDDHA Iron and it turned my water dark red. I added 1/2 teaspoon to 80 gallons of water, and I could no longer see the bottom of the tank. It took over (10) 20% water changes to get clear again. Did you ever find a good EDDHA Iron, preferably one that could be ordered online from a store like amazon

    • Yes, we’re getting quite a few reports of this. We’re looking around and will let you know when we have an EDDHA supplier. In the meantime you can use DTPA. I know that the Aquaponic Source is working on this issue too, so definitely check out their website.

  7. Why does my soil garden grow without the addition of iron while my aquaponic garden requires constant supplementation? Is it the lack of humus in aquaponics or the lack of an intact web of microorganisms that makes the difference?

    • Well Bob, that’s a complex question. Many soils actually do have iron deficiencies which are treated with chelated iron. It has less to do with the humus and more to do with the source material (decaying rock and minerals in soil, and fish feed in aquaponic systems). Sometimes iron is very deficient in nature as well. In these scenarios, plants do some pretty amazing things to get iron into their systems (often times there’s plenty of iron, just not soluble iron). So, in the absence of these plants or the presence of heavy iron consumers, we have to take things into our own hands and add iron.

  8. how about ironite brand mineral supplement? doesn’t say which type of iron it is. only one they carry at ace and home depot around here. ever heard of it?

    • Yes, Ironite isn’t chelated iron- it’s an iron sulfate product if I recall. It can be used, but won’t be as effective as chelated iron, and also contains other things like arsenic that you don’t really want to be adding to your system- whether they’re bioavailable or not.

    • disregard. closer inspection reveals that it is EDTA

  9. Anyone find EDDHA that does not turn water red? Thanks!

  10. Hi Nate, I’m just a simple minded idiot who owns a commercial construction company and builds Marriots and things. Your videos are way too complicated for me, and my brain is like a shelf, not a storage unit, and when i am trying to store the info you give all kinds of “important” things that the little woman told me are dropping off the back of the shelf. My biggest problem is that i cannot figuere out what product you use. Can you look at your bag and tell me the name etc. so i can go do it and then get my daughter to figure out how much to put in my 350 gallon system? Please help.

    Mr. Moron

  11. sequestrene 138fe is eddha. bought a 5 lb. bag. turned water deep red. couldn’t even see the fish anymore. gotta be a better way.

  12. Hi there Nate!

    I am very interested in using barley or other grass species for producing chelating agents. What more can you tell about that? in one of your videos I have seen that you talk about growing barley as insulation for a building. Did you do this? With those quantities of barley it would be interesting to measure the value of chelated iron in the water.

  13. Wow…it was like going back to chemistry class again :)

    But it was great though. Don’t think I have ever read so much on iron before. Thank you for sharing this Nate. I sure know a lot more about iron now.

    Thanks again.


    • Awesome. Thanks Steven. I’m glad it was useful to you!

  14. Yes I use the same and my water is red, half a teaspoon in 150 Gallon turns it dark red

  15. I’ll put it in my fish water so it will hit the grow bed with the flood when there will be less oxygen to make it rust.

  16. I am wanting to make sure I am not dosing the water with too much Iron so to ensure my fish’s safety. It appears all iron tests are not created equal and only some iron tests will test for EDDHA and some will only test for DTPA and some will only test for non-chelated iron. Also, from the research I have done, it appears that most common aquarium iron test kits only test for chelated EDTA and non-chelated iron such as ferrous sulfate.

    Is there a test kit that anyone could recommend when using Fe(DTPA) and Fe(EDDHA) ??

  17. Hello Nate,

    Im from VietNam, its not easy to buy Fe DTPA at here. If i ship from amazon, its to expensive for commercial AP project. Can we use some Fe source that is available in nature?

    • Hi Van Le,
      Your best bet for iron supplementation is to use chelated iron, but in its absence, you can do a few things. Some folks have had luck with an anaerobic tank for reducing iron in solution, but I think the best bet is to plant natural chelate-producing plants in your system. Rye is one of these plants. It probably deserves more research, but there have been excellent results with rye intercropping in soil systems. I don’t see why it wouldn’t work in an aquaponic system.

      • Thanks Nate, i will try with anaerobic tank to see Iron content effect and come back to you with results.

      • Hello Nate,

        If we put red worm in media begs to eat fish waste and release nutrient, does systems will get enough Fe? or we still need to use Fe chelated?

  18. Hello Nate, thanks for your post.
    Can i ask you one thing?
    For 1 ton of veggies. How many weight of Fe DTPA we need in estimation?

  19. Hello Nate,

    When system at low pH, it means nitrification bacteria is working well, and it mean this must be aeration, not anaeration?
    When system at high pH, it means denitrification bacteria is working well, and it mean this must be anaeration.

  20. Well, today I had the sad realization of red water after adding a teaspoon. Can hardly see the fish!
    Iron chelate EDDHA from Combo also turns you water deep red! Even non lethat it’s just impossible to submit the fishes to such an environment!

    Cheers! :)

  21. Today I got Fe7 EDDHA from Solufeed, UK, and guess what? :) Yes.. red water! Anyway, I got used to it, and the darkening of the water helps to keep the algae concentration down :)

  22. I love that Aquaponics can be done on a large scale but also on a smaller scale with children to teach them about the environment as well as spending time together. We have a video of a couple who have recently started their own Aquaponics; it may be helpful to readers who are interested in starting their own. The couple works with schools to have children visit to learn more about growing healthy food. http://bit.ly/1vUpI7d It’s an awesome video with a lot of information of how they got started and what they do.

    • Pretty cool stuff, Shirley! Thanks for sharing. You might look into our growing list of Upstart Farmers as you look for future stories of sustainable farming.

  23. I just added 1/2 ounce of Southern Ag FeEDDHA in about a 400 gal system. It is now totally dark red. Have I destroyed my 10 week old tilapia?

    • David! You should be just fine. Red water might seem a bit disheartening, but everything should be okay. How’s it going now?

      • Thanks for the reply. It’s been a week and a half and the fish seem fine, a big relief. The plants in the grow beds now look fantastic! I’ll probably use a different iron in the future, though, when needed.

  24. Does chelated iron have expiry date? This question is because I use very little quantity, and the bag will take long time to become used up

    • Hi Diego! You should be fine. Chelated iron supplements usually don’t have an expiration date.



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