What about strontium?

Strontium ions also resemble calcium, and like magnesium, strontium is taken up as part of the calcium carbonate structure. However, there has never been any documented need for strontium for anything commonly kept in reef aquariums. It does not perform an important function as magnesium does. The simple fact is that strontium happens to resemble calcium enough, that it is taken up like calcium.
If there is any need for strontium in reef aquariums, there is an abundant amount in aquarium salt mixes.

Quoting Dr. Randy Holmes Farley;
        
“Strontium may or may not be useful in coral reef aquaria, and it may or may not become depleted. Like magnesium, strontium becomes incorporated into calcium carbonate in place of a portion of the calcium ions. That incorporation happens to approximately the same extent whether a coral skeleton is being formed or an abiotic (nonbiological) precipitate on a pump’s impeller. It is just a fact of life that strontium looks a lot like calcium, so it gets into places where calcium would otherwise be. Some aquarists have concluded that strontium may help corals to deposit their skeletons despite the absence of any evidence of this in the scientific literature, and of any direct experimental evidence (for or against) by hobbyists. “

“A few years back, when I tested my aquarium’s water for strontium (using a sophisticated lab machine), I found that in my reef aquarium, with no recent strontium additions, strontium was already elevated (15ppm) above natural levels (8ppm). By testing the Instant Ocean salt mix that I was using, I found that it, too was elevated (15ppm). I saw no evidence of depletion, at least not when performing my routine of changing 1% off the tank’s water daily. I would not like to see the strontium level get any higher, because strontium is known to be toxic to some marine organisms at levels not too far above that. Consequently, adding a supplement without knowing the aquarium’s current strontium level is not advisable” Farley, Reef Keeping 2007-0