I've been dumping about 1/4 to 1/2 teaspoon of pickling lime into my tank each night. It dump it in full dose after I swoosh it around in a cup of cold water. My reef system is around 140 gallons of salt-water.
At 7 A.M. my pH was 7.9

At 8 P.M. (when the tank light is still on) my pH is at 8.1

5 minutes after the light goes off, my pH is at 7.69

How low is to low for pH measurement?

This is a new unit. I did not adjust the tempurate when I calibrate the pH last night. I just adjust the reading dead on to 7 at 10 even the room temp was at 72F.

Should I do any kind of intervention to raise the pH to 8.2?
I have the equipment to do it. But I haven't got around to set up the Kalk to drip slowly at night.