Mercury in a marine aquarium and its importance

Table of Contents

What is MERCURY (Hg)?

Mercury (Hg, Latin: hydrargyrum) is an element belonging to the group of transition metals* and is the only metal that exists in the form of a liquid under normal conditions. Mercury is found in small amounts in the earth’s crust in the form of native mercury (Hg) and as a component of minerals such as cinnabar (HgS) and calomel (Hg2CL2). Mercury is highly volatile, so it quickly enters the air, becoming a poisonous gas.

The importance of mercury in seawater

Mercury doesn’t play a significant role as a trace element in seawater; instead, it’s toxic to living organisms. Its presence in a reef tank should be considered pollution. Higher concentrations of mercury, detectable by ICP-OES devices, are toxic to corals.  

Dangers related to excess mercury in seawater

The presence of mercury in seawater is undesirable and toxic to marine animals. High levels of mercury can lead to health problems in corals and other animals, and even cause their death.

How to protect your aquarium?

It’s important to regularly monitor the mercury content and maintain it at an undetectable level – 0 μg/L. Such a level of mercury in seawater ensures the health and vibrant coloration of marine animals.

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

Indicators of abnormal mercury levels in a marine aquarium

No species of animals or algae sensitive to mercury overdose, known as indicators, are known. An excess of this trace element negatively affects most animals.


To maintain the proper level of mercury in the aquarium, you should regularly test it. If the mercury level remains high for an extended period, it can lead to health issues for corals and other animals, even causing their death. The most common reasons for exceeding the recommended mercury level are:

  • contaminated salt,
  • paints,
  • thermometers.

Find and eliminate the cause of the problem, then lower the value of this parameter in the water. Perform up to 6 water changes. It’s recommended to replace about 15% of the aquarium water during each change until the recommended value of this parameter is reached. 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.


The presence of mercury (a heavy metal) in seawater poses a serious threat to most marine organisms because even small amounts can harm animals. To protect marine animals in the aquarium from the negative effects of mercury, it’s recommended to maintain its level at an undetectable value (0 μg/L), ensuring the health and vibrant coloration of the animals.


*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 :))