I haven't had much luck with the Hanna Checkers. Maybe the phosphate one is OK. But Calcium NO and Alkalinity no. I was sent off on wild goose chases trying to fix problems that did not exist. I eventually settled on Salifert and Red Sea. I used both to confirm the readings.
My suggestion is, if you get readings that seem off, don't go crazy. Try and find a way to confirm your readings before acting.