plants in the aquarium

Iron (Fe) in the aquarium

Over the years, a lot of myths about iron (Fe) in the aquarium have been created. Therefore, many aquarists do not consider the necessity of dosing this element into the water or are simply afraid of it. I also see this tendency among my clients in my everyday work. For this purpose, over the past two years I have tested a number of compounds (salts) containing iron (Fe) in the aquarium under various environmental conditions (varying pH values, different dosage forms, different lighting). This allowed for an in-depth analysis of the iron problem as a limiting factor in the growth of not only plants but also algae. I shall present some of these findings in the following article.

Iron reactivity

Iron (Fe) by itself in aquatic conditions (rivers, lakes or home aquariums) does not have an easy life. It is extremely sensitive (reactive) to oxygen, and yet every aquarist wants there to be enough oxygen in his tank so that the fish do not have breathing problems. Unfortunately, oxygen is not the only problem that iron (Fe) has in an aquarium, as it also forms many salts that are practically insoluble in water. For many aquarium plants, iron (Fe) is only available in the second oxidation state in the water, so its high reactivity can be problematic.

What is iron responsible in plants for?

Iron is essential for enzymes to catalyze many chemical reactions occurring in plants. A catalyst, on the other hand, is a substance that makes it possible for a reaction to take place under much milder and more efficient conditions, so that the plant saves the energy necessary for life. In chemistry, such substances are called cofactors.

Because of its very efficient oxidation state change from Fe2+ to Fe3+ (and vice versa), iron is great for electron transport. In addition, like in humans, it is necessary for plants to respire – as a necessary component for the action of cytochrome in the cell, a protein used in the respiratory cycle.

Why is there so little iron in a tank?

Iron is all around us. How does it happen that it is always missing in the aquarium? In water, iron forms insoluble oxides, hydroxides, salts and readily combines with DOC ( Dissolved Organic Carbon). Formerly, iron(III) chloride FeCl3 was commonly used as a coagulant to treat wastewater and water of excess phosphate (PO43-) according to the reaction: Fe3+ + PO43- → FePO4↓ , where an orange or brown precipitate is formed, often observed on sedimentation (string) filters, very popular as pre-filtration in home plumbing.

How do plants take up iron?

Plants growing on the surface of the earth usually have no problem with access to iron. As it is necessary for them to live, they have developed very clever ways of obtaining it. Using their root system, they acidify the soil around them with H+ ions to increase mobility and change the oxidation state from Fe3+ to Fe2+. An even cleverer way is to secrete (into the soil surrounding the roots) complexones that absorb Fe3+ and transport the iron into the plant. Aquatic plants with extensive root systems benefit in similar ways, but the constant presence of a solvent like water sometimes becomes dangerous. The biggest problem is therefore the acquisition of iron by stem plants, for which the roots are only a clinging organ (anchor them to the substrate), but they take nutrients from the water.

How to recognize iron (Fe) deficiency in plants?

Iron is essential for plant processes. In addition, it is not a mobile compound, so in the case of its absence in the newly formed parts of the organism, the plant is not able to transport once used iron further, so deficiencies will be observed in freshly growing leaves, the fastest in fast growing plants. These deficiencies are manifested by leaf blanching and dwarfing. A similar effect occurs in the absence of manganese (Mn), but here we are also dealing with the loss of the growth tip of the plant. To make control easier I have always planted common hemianthus (Hemianthus micranthemoides) in my tank. I noticed that it reacts very quickly to changes in iron concentration in water, so it works perfectly as a kind of living indicator. The same effect can be achieved by planting e.g. waterweed (Elodea canadensis).

How to recognize iron (Fe) excess in plants?

As you can already deduce from the above information, in a well-functioning and stable tank it is quite difficult to create a situation in which the water is too saturated with iron ions. But it’s different with the substrate. The development of a plant that forms an extensive root system can be stunted if there is too much iron in the environment. Wendt’s water trumpet (Cryptocoryne wendtii ‘brown’) covered for the test with a large amount of clay balls with high iron salt content started to lose its leaf blade after a week. When it was taken out of the medium after 16 days, its root system was irreversibly damaged.

Water conditioners and iron (Fe) in the aquarium

Let’s assume that we have conditions in the tank that allow iron ions to exist in free form in the tank. Can we stop worrying? Not really. Aquarium owners use water conditioners to prepare tap water for use in aquariums. Among other things, they remove chlorine and heavy metal ions. Currently, the most common additive used in conditioners is EDTA (ethylenediaminetetraacetic acid, versic acid). This additive itself is poorly soluble in water, so its sodium salt (sodium edetate) is used. EDTA is a complexone. To fully understand the action of this substance, it is necessary to comprehend its different affinities for individual ions. One ion will react better, another worse. This relationship is presented in the table below.

EDTA affinity for different ions


The higher the logβ value in the table, the greater the potential for EDTA to “absorb” the cation. The table therefore shows why iron in the form of Fe3+ can only be fed to the root system. It is not only the formed oxides and insoluble salts that hinder it. Adding EDTA immediately masks the ion even before it absorbs the heavy metals we want to get rid of first, such as copper (Cu), lead (Pb) or zinc (Zn) and aluminium (Al). Individual ions can exchange in the EDTA molecule, replacing the ion with a weaker affinity with the stronger one. What if you didn’t use conditioners? EDTA comes from more than just this source, as we will see later in this article. And water has to be treated anyway.

Ways of supplying iron to water

In order to supply iron (Fe) to the aquarium in a bioavailable form, you need to find compounds that are well soluble in water, but still hold the ion in their structure, that will not eject the iron from the molecule because it will immediately combine with something that will knock it off. And here previously mentioned EDTA comes to the rescue together with citrates or gluconates and many other compounds.


Most of us don’t realize how many things in our daily lives contain EDTA in their ingredients. Starting from laundry detergents, to preservatives in cosmetics and medical applications. Such diverse use generates pollution of anthropogenic origin (for which man is to blame) and may disrupt the natural circulation of metal ions in nature. EDTA complexes are degraded by light (photodegradation) and can also be degraded by bacteria under appropriate environmental pH conditions.


These compounds were not so long ago very popular among aquarists. They are formed by the Krebs Cycle, among other things, so everyone who had biology class has heard of them. These salts are very soluble in water, but are rarely used today. You have to synthesize iron citrate (actually sodium-iron) yourself from the sodium salt, which further complicates things.


Salts formed by reaction with gluconic acid. Nowadays it is widely used in medicine, mainly due to its highly absorbable calcium gluconate. Used in aquaristics, iron (II) gluconate is ideal for quick supplementation in aquariums with low levels of this metal, in soft or moderately hard water, with pH up to 6.5-7.0.


Dissolved organic carbon is formed in every aquarium. It is a kind of natural protection of the tank against heavy metals present in water. Combining with iron, it constitutes a store of this element occurring in the water-depth and in the substrate. As it decomposes over time, it releases small amounts of iron into the water – just in time for ongoing plant consumption.

Which form of iron (Fe) is best to use in tha aquarium?

Unfortunately, the answer to this question is very difficult. From the previous discussion, it is evident that there are constantly some amounts of iron “circulating” in the water associated with EDTA and DOC, which are natural reservoirs of this element. Most manufacturers do not state what substance is in the fertilizer they offer. We usually only get information about “iron chelate”. This is why it is so important to observe the tank. If the plants do not show any visible signs of its absence, let’s apply substances that will calmly, at their own pace get into the water. For large and sudden deficiencies, consider using easily soluble salts or gluconate.

Limiting agent

We already know that iron (Fe) is essential in an aquarium, and its lack effectively blocks plant growth. This unequivocally leads the conclusion that it should be supplemented into the tank. However, it is worth doing carefully. I wrote about an example of an attempt to suddenly influence an important parameter in an aquarium in the article The pH buffers in the aquarium. If we find that there is a large accumulation of nitrates and phosphates in the tank, we should dose iron very carefully, as algae, being simpler organisms, may start to use it first, effectively making our life miserable. In this case, first consider reducing the amount of NO3 and PO43-, especially considering that with the latter ion iron forms a poorly soluble precipitate as mentioned above.


Using the previous information, we can plan to supplement iron deficiency. Observing and measuring the decreasing concentration of Fe ions, we have to decide how to dose them. We can use readily soluble salts to raise its level instantly, but at the same time we must be aware that some of the ions will go into insoluble forms, binding to phosphates or reacting with oxygen and DOC. At the same time, we can create a “store” of iron in the substrate for plants that prefer to obtain it through the root system. It’s all up to us, and the ultimate goal we want to achieve is a beautiful aquarium.

Michał Wyskiel

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