Originally Posted by tsunami
Perhaps Ca/Mg relationship is the culprit and not actual hardness? Hmm.
That's what I was thinking as well. Separate accurate measurements of Ca/Mg require expensive kits, so I am not sure what exactly the values are in my tap. I am also doing an experiment with partial RO with small Ca addition to see how it turns out for me, so it will be softer water with higher Ca/Mg ratio(actually it might be any other elements in tap water, like Ca/TDS? because some people report Ca/K also matters, but not in my case). But it is a low tech tank w/o CO2 so it would take a long time to see the effects.