A CH of 450 is starting to be a bit much. In pools we might manage it (compensate) with a lower pH. In a hot tub however, it may simply be easier to swap the water. Perhaps time to do an Ahh-Some cleaning and start fresh? Curious though …. is your local MN water that hard, or is the CH increase due to constant use of Cal-Hypo? Can you also confirm for us how you are testing the CH? Which test kit are you using?
Yikes. That water is southwest territory-worthy. :bowdown: Okay, then two things to consider ..... replace with some soft water is you are able. Also watch the pH to ensure it doesn't spike to high which if it does would tend to add to the CH's ability to cause scale over time. With the 2006 kit, do you also have the magnetic speedstir? If not, you might consider getting one from TFTeskits.net. The speedstir has been known to actually show a slightly lower CH result due to its efficient mixing.
The good news is I have yet to fill my tub, so the world is my oyster. I do use a magnetic stirrer, and that would make sense why my reading is a bit lower than the conversion of grains per gallon to ppm as supplied by our city.