Magnesium is insoluble in limewater, precipitating as magnesium hydroxide.
As an aside, that's one reason that the Brightwell product Kalk+2 is so misleading. The second is that, but its own claim, it hardly has any magnesium in it anyway, less than my pure food grade calcium oxide bulk material did.
As another aside, I'm interested in adding a bit of kalk to my top off water, to supplement but not replace my two part dosing. I'd like to avoid issues with precipitation on the bottom of the topoff reservoir, damage to the submerged pump, clogged lines, etc, if possible. What dosage or dosage range of kalk per gallon H2O would be right for this approach?