Iron in marine aquarium and its importance

Iron in marine aquarium and its importance

Table of Contents

What is IRON (Fe)?

Iron (Fe, Latin: ferrum) is a metal that belongs to the group of transition metals and is located in the eighth group of the periodic table. It has a shiny, silver appearance.

It’s one of the prominent elements found on Earth, ranking fourth in abundance. Iron is a component of both the Earth’s outer and inner core. In the Earth’s crust, it mainly exists in the form of minerals such as hematite (Fe2O3) and magnetite (Fe3O4).

Iron is commonly found in a +II oxidation state, for example, in iron(II) sulfate (Fe(SO4)2), and also in a +III oxidation state, as seen in iron(III) chloride (FeCl3). Iron in its pure form in seawater is not stable, similar to manganese. It readily reacts with other compounds and can precipitate as visible sediments.

Importance of iron (Fe) in seawater

Iron is a crucial element for all plant and animal cells. Basically, every living thing needs iron. It plays a significant role in metabolic processes and important enzyme-controlled activities. Iron is involved in so many processes that listing them all here is impossible.

Another important aspect is the photochemistry of this element – light-induced changes in the oxidation state of iron. Scientific studies suggest that light affects the bioavailability of iron, especially for phytoplankton.

Iron is a vital trace element in reef tanks. In natural seawater, iron is limited, and at concentrations of just a few nanograms (10-9g) per liter, it’s almost undetectable.

Natural sources of iron for coral reefs include bacteria, phytoplankton, and food. In many cases, it’s iron, not, as often read, nutrients like PO4³⁻ and NO3, that is a limiting factor for coral reefs. Therefore, it’s important to replenish iron deficiencies. The level of iron in the tank should be maintained in the recommended range: 0.1-1 µg/l.

The role of iron (Fe) in marine aquariums

Iron is a fundamental element for all cells of marine animals and algae. Iron, along with nickel and zinc, participates in the processing of nitrogen compounds by bacteria. Controlling the level of iron in a reef tank is crucial because it can have potentially harmful effects.

The role of iron – the photosynthesis process

Iron plays an incredibly important role in the process of photosynthesis, which is a key mechanism sustaining life in marine ecosystems. It determines the amount of chlorophyll, a substance responsible for the green color in plants and photosynthetic organisms. Chlorophyll’s job is to absorb light and convert it into chemical energy essential for the life of marine animals.

The role of iron – coral pigment

The presence or absence of iron affects the coloration of corals. However, it’s important to remember that adjusting the amount of iron in a reef tank can have varied effects, depending on the species and type of corals in the aquarium. In some cases, increasing the iron level can contribute to enhancing red, pink, and green colors.

The role of iron – nutritional function

Iron deficiency can affect organisms’ ability to absorb other micronutrients, leading to further health problems. Low iron levels can disrupt chlorophyll production.

The presence of iron in water promotes the growth of phytoplankton. Iron also plays a crucial role as a nutrient for algae. For them, iron is essential for carrying out the process of photosynthesis and producing their own organic compounds. Monitoring iron levels in this case is important due to the risk of uncontrolled algae growth.

The role of iron – biological function (enzymatic processes)

Iron also plays a role in nitrogen availability. If there’s not enough nickel and zinc along with iron, corals and biofilms may struggle to absorb or process nitrogen compounds. Usually, the nutrient levels in the system significantly increase in such cases.

The role of iron – protection against parasites

Not having enough iron can make marine organisms weaker, making them more prone to diseases and other stressful factors. On the other hand, having too much iron in the tank can expose animals to parasite attacks and also slow down the growth of bacteria and algae.

Problems related to deficiency and excess of iron (Fe) in seawater

Low iron levels can lead to various problems, especially if the inhabitants of the tank need this element for proper development. Using zeolites in the aquarium system increases the removal of iron, so these tanks may require additional dosing. A lack of iron in the water can worsen the metabolism of corals, negatively affecting their health, growth, and coloration. It can also lead to nitrogen-related issues in the aquarium.

High iron concentrations have a comparable effect to over-fertilization. Corals are adapted to limited iron, and they struggle with consistently high levels. Excessive iron can easily lead to darkening of corals and promote algae growth. However, it’s essential to remember that an excess of iron can be toxic to many marine organisms, especially corals.

How to protect your aquarium?

It’s important to regularly check the iron levels and keep them at the right amount. We recommend maintaining iron at a level between 0.1-1 μg/L, with the optimal value being 0.2 μg/L. Keeping iron in seawater at the right level ensures the health and beautiful coloration of marine animals.

The most accurate and reliable way to measure iron is through ICP-OES analysis. Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) is the most precise analytical method for determining the elemental composition of seawater.

Indicators of abnormal iron (Fe) levels in a marine aquarium

Green and yellow corals are the best indicators of the iron level in a reef tank. A great indicator of the right iron level is, for example, Acropora tumida, which has a bright green color when there’s enough iron in the seawater. Hydnophora and many other LPS corals only show polyps when there’s sufficient iron supply. Most corals lose color when the iron supply is too low.


  • Green color fading in SPS corals

  • Trouble absorbing nutrients

  • Stopping the creation of chlorophyll


  • Death caused by iron toxicity at high concentrations

  • Darkening of corals

  • Algae bloom

  • Cloudiness in the water


To maintain the right level of iron in the aquarium, you should regularly test it and ensure it stays at the proper level. If the iron level goes above 1 µg/L, it’s considered exceeding the recommended content in the reef tank. 

The most common reasons for exceeding the recommended iron level are:

  • Contaminated salt,

  • Rusting pump impellers,

  • Rusting stainless steel elements (including frames used as aquarium supports),

  • Foods like brine shrimp,

  • Excessive doses of administered liquids.

Find and eliminate the cause of the problem, and lower the value of this parameter in the water. Perform up to 6 water changes. It is recommended to exchange about 15% of the aquarium water during each change until the recommended value of this parameter is achieved. The water prepared for replacement must have the appropriate target salinity level. Use salt with the correct parameters and composition suitable for conducting the ICP test.

If the iron level is below 0.1 µg/L, we recommend using products containing this element to balance its level. Iron itself is not stable in seawater. It always has a limited presence concerning coral reefs because keeping it dissolved in seawater is a challenge. Natural chemical and physical processes, such as oxidation and adsorption, result in limited iron availability for marine organisms. Therefore, proper coral feeding is crucial because food provides them with essential elements and minerals, including iron.

To ensure a constant iron level in a marine aquarium, we recommend systematically supplementing this element based on the coral population in your tank.

*Transition metals – a group of chemical elements in the periodic table, including the side groups of the periodic table, i.e., groups 3-12.

About author

Picture of Magdalena Metzler

Magdalena Metzler

Privately, I am a mother and a lover of nature and sport. My main interest is quantum chemistry, which hides a whole lot of unsolved mysteries and connections, which is extremely exciting from a scientific point of view.
In my scientific career, I have conducted international projects focused on innovative solutions for many branches of business, e.g. automotive, construction, and now, of course, marine aquaristics.
Working at Reef Factory gave me a passion for marine aquaristics, which I can develop every day, building a chemistry department and creating products that will help aquarists take care of tanks and ensure the highest safety of animals. One of the most exciting memories of working at Reef Factory is the commissioning of the ICP-OES spectrometer, which analyzes the elemental composition of seawater. The method of analysis in ICP is based on an analytical technique, which is a combination of my passion for quantum chemistry and marine aquaristics.
I hope you find my articles on ReefPedia interesting and helpful! Happy reading :))