Trace Elements


For us to continue to grow corals successfully, we need to pay close attention to our water chemistry. In practice, we routinely test for major elements such as calcium, alkalinity, and magnesium but we don’t often test for trace elements. Why is that? Are they not important? On the contrary! They ARE important. So let’s take a closer look.

What are trace elements exactly and what role do they play in our reefs? To put it simply, trace elements are elements that appear in very small quantities in salt water. Literally trace amounts. They are vital to all sorts of biological processes and due to the limited size of our aquariums can be depleted rapidly. In addition to being soaked up by the organisms in your tank, trace elements can be exported by protein skimmers and chemical filtration.

Adding trace elements is really easy. Trace elements can be replenished through regular water changes or with chemical additives. They can also be introduced from foods including amino acid supplements. It turns out that a lot of brands of amino acids don’t just contain amino acids, they are bundled with trace elements. Similarly, chemical additives for major elements like 2 part calcium and alkalinity often contain trace elements as well. Most of the time though, people can get by with just doing regular water changes but a stand-alone trace element additive is also popular for those looking to supplement their levels. And THAT brings me to my next point. It is very easy to add trace elements BUT before you run out and start dosing trace elements, it is important to realize just how scarce they are in our reef.

 Composition of Saltwater

Let’s take a look at the composition of salt water. Saltwater with a specific gravity of 1.025 is made up 96.5% of water. That remaining 3.5% is the difference between fresh water and salt water. This is what we will call the sea salts. Let’s take a closer look at that 3.5%. It is made up of the following major ions:

You can see that the major elements comprise the vast majority of “sea salts.” If you were to remove those major elements from the mix, what is left is a whopping 0.2%. Remember that this IS NOT 0.2% of the total composition of natural sea water, we are just talking about 0.2% of that 3.5% that ISN’T literally H2O.

In total, there are around 70 different trace elements in salt water and all together make up that 0.2%. I won’t cover all 70 but I’ll touch on the top 5 that come up frequently in fish tank discussions just to give you some background.


Let’s start with Iodine. Iodine is a trace element that has a long history of anecdotal observations attached to it. I have heard that it benefits soft corals, gorgonians, tunicates, sponges, and macro algae as well as molting invertebrates such as shrimp and crabs. Natural sea water has an iodine concentration of between 0.025ppm to 0.08ppm depending on location. Iodine is an unstable element in a home aquarium and can take on multiple forms such as molecular iodine (I2), Iodide (I-) or Iodate (IO3-). It is a toxic substance in its I2 form and this toxicity is why it is often used as an antiseptic dip. For example, Lugol’s Iodine solution which is a very good antiseptic is a mix of molecular iodine (I2) and Iodide (I-). We use it here as a dip when we suspect a coral is having a bacteria-related issue.

Iodine enters your system through salt mixes, feeding (particularly algae based foods), and supplementation. Iodine can be removed from your system by protein skimmers, chemical filtration, algae scrubbing, and the organisms in reef using it up through their normal biological processes.

You will get a lot of differing opinions about whether it is necessary to dose Iodine in your aquarium. Most hobbyists believe that all the organisms get their requirements met by regular water changes and feedings. My take on it is if you plan to dose Iodine, it is important to test for it. That goes for every trace element really. Blindly adding it can be problematic because trace element concentrations in levels might higher than what is found in the ocean can be toxic, and we’ve already discussed that it doesn’t take a lot to be a lot higher than ocean concentrations.

Having said that, testing can be a little tricky. Depending on what form Iodine takes, it may or may not show up on a test kit. It is not always clear what exactly a test kit is detecting. Some detect only one of the forms while others detect a combination. The only test kit we have experience with is the Salifert test kit which tests for both Iodide and Iodate. Another option is sending a water sample off to a lab and having it ICP tested.


Iron plays a big role in a number of biological processes. In animals it is used in oxygen transport. If you have ever heard of hemoglobin in red blood cells, “heme” refers to iron, and thus hemoglobin is an iron-based molecule that is used to transport oxygen. In algae and corals, iron promotes photosynthetic activity. Photosynthesis splits carbon dioxide into carbon and oxygen resulting in the simple sugar glucose and oxygen gas. This is a multi-step process and all along the way there are electron transfers facilitated by iron.

In the ocean, Iron promotes photosynthetic activity in algae, both for zooxanthellae in the corals and macro algae. That’s why sometimes you hear iron come up when hobbyists discuss algae filtration systems that rely on rapid growth and harvesting of algae. The growth of algae in productive refugiums can strip out the trace quantities of iron in the water and the algae stops growing.

Iron is looked at as a hard limiter on the growth of algae. We often associate algae growth and phosphate levels in the water, but a much more relevant measure is the iron content. You definitely don’t want to over-do it though. While iron is not thought to be toxic to our aquarium inhabitants when overdosed, it can lead to major algae issues. When iron levels get out of control is when you normally see catastrophic algae blooms in the reefs.


The next trace element we can talk about is Strontium. Strontium acts a lot like Calcium in that it is taken in by stony corals in our reef tanks and incorporated into their aragonite skeletons. Some aquarists observed that adding strontium helped increase stony coral growth rates, but the jury is out on that.

What we know for sure that it happens but it is unclear whether the incorporation of Strontium into the skeleton is a good thing or not. It could be a positive mechanism to accelerate growth but it could also be the coral trying to isolate a potentially toxic metal by depositing it in Calcium Carbonate.

Strontium gets introduced mainly through salt mixes. Some brands of sea salt have doubled down on its possible beneficial effects and have chosen to formulate their product with elevated levels of Strontium.


This next trace element is one that does not have a ton of research studies behind it, and it is frankly not something that was on my radar at all until somewhat recently. This trace element is Molybdenum. It is considered an essential trace element for both animal and plant health as it plays an important role in the functioning of a number of enzymes as well as chemical reactions involving nitrogen, carbon, and sulfur.

In reef systems, Molybdenum is thought to assist in the conversion of nitrate to nitrogen gas as well as promoting the growth of zooxanthellae in corals.

Like with every other trace element on this list, there is such a thing as too much of a good thing. Molybdenum is toxic in high concentrations.


The last trace element I will cover in this video is Manganese. There was a study done in 2018 looking at the function of Manganese in coral biology and it appears to have something to do with the coral’s ability to resist stress. In particular it looked at whether corals bleached when stress was introduced such as heat. The study showed that the corals provided with manganese resisted bleaching and the reason is thought to be Manganese helps break down the formation of oxide radicals in the body. It is literally an anti-oxidant.

The other effect that Manganese had in the corals is it turbo-charged photosynthesis in the zooxanthellae. Manganese had been shown previously to improve the photosynthetic productivity of phytoplankton and algae but it turns out that it also does so in corals.

 Should You Dose Trace Elements?

In summary, trace elements are vitally important to all sorts of biological processes that happen in our aquariums. So the question remains, should reef aquarists be dosing trace elements regularly?

It depends on the uptake of trace elements by your reef's inhabitants and the frequency of water changes. Most modern salt mixes today have more trace elements than natural salt water in anticipation of uptake by tank in habitants. In most cases, weekly 10% water changes are more than enough to replenish trace elements.

It is possible though that heavily stocked tanks will deplete trace elements faster than sparsely stocked aquariums. In this situation, a trace element supplement could help. My rule for dosing anything is pretty simple. Don’t add any supplement that you aren’t testing for. It is important to know where your levels are before you start adding and know when you should stop. Blindly adding iodine because someone online said it was good for Xenia is going to lead to more problems. Excess trace elements in concentrations substantially higher than natural salt levels can be toxic and we already talked about the vanishingly small quantities they represent in our salt water so it doesn’t take very much to overdose.

Than Thein

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