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, AquariumWaterTesting.com 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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