Maybe if you have small nubs, you can look at their growth rate over several days, but when your acros are the size of basketballs, you will never notice if it stops growing. My "canary in the coal mine" is my derasa. If the mantle pulls in, I know to go check what's going on. It's usually alk or pH, but those 2 are closely linked. And since I run a calcium reactor, I don't really check calcium much anymore. Maybe a few times a year. Alk and magnesium are the 2 I test for most frequently. The rest are just occasionally for curiosity sake. I have a Milwaukee pH controller, so I technically test for that continuously, because all I have to do is look at the reading on the monitor.