I don't think doing waterchanges is your main concern right now. I would be getting test kits right away. Buy good ones - Salifert or Elos. The Hanna Checker for alkalinity is really good (the calcium one is terrible imo).
Step 1 - test magnesium. If it is lower than 1350, raise it up around 1300-1500 with magnesium chloride.
Step 2 - test alkalinity. Dose if needed (8-9 dKH). If high, move to step 3. Don't worry if it's 11-12 dKH, that's fine, just make just your calcium matches that (see chart below).
Step 3 - test calcium. Dose if needed (415-430 ppm). If high, wait it out.
Honestly, I think the problem is that you have high calcium and alkalinity and low magnesium. When you get magnesium up higher it should sort itself out. If it's bacteria, it will sort itself out too.
Also, test calcium, alkalinity, and magnesium of your water change water every time you open a new bucket. I haven't noticed an increase in magnesium in IO yet - maybe I have an old batch. If you're doing a bunch of water changes with saltwater low in magnesium it's not going to magically raise magnesium.
