How Important are Trace Elements in the Reef Aquarium?

24 Oct, 2014

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. They are vital to all sorts of biological processes and due to the limited size of our aquariums can be depleted rapidly. Trace elements can be replenished through regular water changes or with chemical additives, but before you run out and start dosing trace elements, it is important to realize just how scarce they are in our reef.

To kick off this discussion, 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. When put in that light, it almost sounds like there is such little difference between fresh and salt water, especially considering that fresh water is not really 100% water it too has its share of trace elements depending on geography. This view of course is faulty. “Sea salts” make up the remaining 3.5%.

That 3.5% sea salt can be broken up into two main categories, major elements and trace elements. The major elements are:
Sodium 30.6%
Chloride 55.0%
Sulfate 7.7%
Magnesium 3.5%
Calcium 1.2%
Potassium 1.1%

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 0.7%. Those are our trace elements. In total, there are around 70 different trace elements in salt water and they all fit into that 0.7%. The 14 most abundant are chromium, cobalt, copper, fluorine, iodine, iron, manganese, molybdenum, nickle, phosphorus, selenium, tin, vanadium, and zinc.

These trace elements are vitally important to all sorts of biological processes that happen in our aquariums. In fact, too little or too much of any of them could cause serious problems. So the question remains, should reef aquarists be dosing trace elements regularly?

In short, 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 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.

If you plan on adding trace elements, make sure you are testing for them especially considering the vanishingly small quantities they represent in our salt water. There are a number of test kits available for testing specific elements. A more esoteric method is to send a water sample off to a professional laboratory and have it tested there [at least two recent introductions, and TRITON US, now offer high-end testing services – Ed.]. Unlike our hobby-level test kits, a professional chemistry lab will have access to devices capable of high performance liquid chromatography (HPLC) or liquid chromatography-mass spectrometry (LC-MS) that are about a million times more accurate than what is sold on the shelf of a local fish store. That might be overkill for a typical home aquarist however large aquaculture facilities and public aquariums regularly send off water to be tested in this manner.

The point is, if you plan to try trace element addition to your reef tank, 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.

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About the author

Avatar photo
Than Thein

Than is the owner of Tidal Gardens and Advanced Reef Aquarium. Than's background is a mix of biology, computer science, business, and law. However, the reef aquarium hobby eventually led him away from a suit and tie corner-office job to pursuing his passion growing coral and shooting underwater videos.


  1. October 28, 2014

    Well said Than. Copper is used by many animals in our tank. DO NOT dump copper into your reef tank! Just about anything can be over done with reef tanks and it is best not to jump into anything new blindly.

    • Avatar photo
      October 29, 2014

      I agree. It is very tempting to try something new that might help your tank, but it is important to understand what is going on and how to avoid doing harm. This is especially true for adding any kind of chemicals to your tank.

  2. October 28, 2014

    Hi Thein,

    my name is Fernando Chang from Panama Republic and just listened your very interesting explanation about the Trace elements in reef tank. Unfortunately I do not dose any trace element in my tank and I am wondering when we change water every week 10 to 15% does this replace the trace elements consumed by the corals and fishes ?

    Fernando Chang

    • Avatar photo
      October 29, 2014

      It really depends on whether you have a lot of corals that drain the trace elements more than is replenished by water changes. If you want to play it safe, just do larger or more frequent water changes.

  3. October 29, 2014

    Interesting article. Important to remember that the 3.5% is 3.5 gm per deciliter and a measure by weight not by concentration. 1.025 is a concentration or about 900 milliosm which is 3 times our own concentration of 300 milli osm in mammals to appreciate how hyperosmolar salt water is. Important distinction since you state sodium is 30.6% of salt which is true by weight but by osmolality sodium as a cation is equal to all the anions, chloride being the main one to achieve electron neutrality. As aquarists, we are more concerned with concentration of salt water and in milli osm. sodium is close to 50% the rest being potassium. It takes a nephrologist to know!

  4. July 30, 2021

    Hi is it ok to use trace elements meant for reef aquariums in fresh water tanks? I know trace elements vary in bodies of water depending on a lot of things but are trace elements between fresh water and sea water very different generaly? I keep fresh water fish that like mineral rich hard water, our rainfall is exceptionaly high so I wish to add trace elements. Only I cant get them for fresh water only salt water tanks. Seems calcium is the highest concentration in these liquid trace elements. I guess oceans are much higher generaly in calcium than fresh water but for the rest Im not sure. Would they be safe to use for fresh water tanks?

    • Avatar photo
      August 02, 2021

      Anton, for freshwater I might suggest looking for products that are meant to reconstitute RO/DI water. Kent Marine offers R/O Right, Seachem offers Equilibrium. Or you can look at additives designed specifically for the water chemistry you’re trying to emulate. E.g. our water here comes out of the tap with a TDS of 43. VERY soft, and with a temporary, artificially high pH that quickly dissipates. I use various additives (salts and buffers) in the aquariums that house African Rift Lake cichlids…without them, they just don’t seem to fare as well (which is entirely understandable).

  5. July 30, 2021

    Think i may have left out my email addy in my message.

    • July 30, 2021

      No not possible lol. Was worried I wouldn’t get notice of the reply

  6. January 04, 2022

    Hey, I would love to put my comment on this blog post!

    Thanks for sharing this content worldwide!

    It is too much helpful for me!

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