I have been keeping reef tanks for 25+ years and during a good chunk of that time I tested calcium, alkalinity, nitrate, pH and salinity on a regular basis and that was about it. I hardly, if ever, tested for magnesium or phosphate and my SPS were healthy and happy.
Was I lucky? Is the minimalist approach the right way to go today in terms of testing? Not necessarily, but is the other extreme a better way to go? Today many folks, including myself, use lab-grade tests like Triton to measure such things as boron, lithium, potassium and strontium.
Triton analyzes the water and gives a hobbyist a complete roadmap on what is lacking, what is on point and MORE