Maximum measurement thresholds

I had to replace my sensor after 3 months and didn't do any Ca etc measuring for a few weeks but assume my ION director was still dosing. A week after the replacement install I noticed my Ca and Mg levels were fairly static with Ca between 404 and 414 even though i was dosing tonreach 440. I followed the Aquacalculator instructions for dosing fauna marin balling salts. The ca didn't move so I continued additional daily dosing over and above my preset daily dosing amounts. I'm now at the point that my gonipora has retracted and my shy serpent starfish is actually visible on the rocks and not under them. I suspect a water chemistry issue. Using both the salifert and aquaforest Ca test kits, neither of the solutions turn pink....I.e. the Ca is possibly >500 ppm. ID says 406. Question: is the probe faulty? Or does it not measure above 500 ppm? I ran the ghl reference solution and the results of the 3rd test run were fairly close to the bottle values....although Ca was under reading by 10ppm (410 ppm). It appears the sensor is working fine. (As a side note, my nitrates won't drop below 22 even directly after a 20% water change, and the Mg won't rise above 1260). Any thoughts? Thanks.