Magnesium?

[mikellini]

What I mean to say is your calcium should not have been that high with such a low magnesium. So either one of the test kits was probably inaccurate. Did you check the expiry? What brand were you using?

Calcium precipitation is more likely to occur near a heat source, like pumps or heaters, but could also occur directly in the water column if conditions are right. Unless your room temperature is higher than the water temp, the glass is the last place I'd expect to find it. Those were probably some sort of snail you were scraping off the glass