Strontium
My recommendation is to maintain strontium levels in reef aquaria in the range of 5-15 ppm. That level roughly spans the level in natural seawater of 8 ppm. I do not recommend that aquarists supplement strontium unless they have measured strontium and found it to be depleted to below 5 ppm. Measuring and supplementing strontium is not a critical activity for most aquarists, and is not a trivial exercise since the available test kits can be difficult to use (see below).
In some recent tests, I found that in my reef aquarium, without any recent strontium additions, strontium was already elevated above natural levels (to 15 ppm due to elevated strontium in the Instant Ocean salt mix that I was using). I would not like to see it get any higher. Consequently, adding a supplement without knowing the aquarium's current strontium level is not advisable. Scientific evidence indicates that some organisms need strontium, albeit not the organisms that most reef keepers maintain. Certain gastropods, cephalopods, and radiolaria, for example, require strontium.18-34 It is, however, clearly toxic at elevated concentrations. In one reported case, 38 ppm was enough strontium to kill a particular species of crab (Carcinus maenas).34 No evidence indicates that 5-15 ppm strontium is harmful to any marine organism, although it is not known what strontium levels are optimal. Finally, anecdotal evidence from a number of advanced aquarists suggests that strontium that is substantially below natural levels is detrimental to the growth of corals that many aquarists maintain, but this effect has not been proven.
How can we maintain natural strontium levels? Doing so, of course, necessitates a suitable strontium test. Some test kits are perhaps suitable for this purpose. If not, sending a sample out to a lab might be a reasonable alternative for some aquarists. If the result comes back in the 5-15 ppm range, no action likely need be taken. If the level is higher than 15 ppm, the best reduction method may simply be water changes with a suitable salt mix, without abnormally high levels of strontium. If strontium levels are below 5 ppm, adding a strontium supplement may be in order.
