I think I know the answer to this, but having a sounding board is helpful.
After testing plumbing for leaks, and fixing them, I filled the sump with a measured volume of with RO water and started circulating it through the pumps and back into sump in an effort to determine amounts of "stuff" needed to bring water to desired parameters. With nothing added I figured I’d test for baseline, make sure I come up with 0’s
The problem is with pH. The controller shows 5.3. The TropicMarin test kits shows off scale in upper range. Checked the probe in calibration fluids and was dead on at 4.01 and 7. Tested tap water with controller and test kit and they were the same. Tested RO water right from unit, controller 5.3, test kit off scale.
Could the test kit be wrong due 0 KH and 0 GH?
As I add “stuff” to the RO water I’ll keep testing and probably answer my own question.