Strontium tests.... hmm, sounds like it tests the measure of Strontium, right?
By doing water changes, it replenishes trace elements such as strontium that might be deficiant in your water column.
It is in coral skeletons and its role is not clear. There is evidence that strontium benefits corals. I am now supplementing it and have not come to my own decision on it as of this time.
Buy the book Marine Chemistry by CR Brightwell if you like this kind of stuff…
Strontium 8ppm at nsw level.Their are product that you can dose biweekly.Also have salt that will replace it after water change. I wouldn't test for it.