You need to start testing and dosing calcium, alkalinity, and magnesium when you add hard corals. You should have tested these and had them in the proper ranges before adding your new corals to reduce stressing them, but hindsight is 20/20.
Imo, the only test kits worth buying for cal, alk, and mag are Salifert or Elos. You can cheap out for your other tests (although a high quality nitrate kit is also essential imo), but not the main ones.
Take a peek at the guide in my signature for testing, dosing, and an easy explanation of these elements.