Kalkwasser

[Aquattro]

I used it for years, loved it. I only stopped due to changing my top off system, and I built my own Ca reactor for very cheap. And kalk can be harder to find, and expensive if you find it. If you can source pickling lime from the US, then it's cheap.
One member here supplied it in bulk, he got it from an Asian source, probably pickling lime.

But with all the new tech in dosing and such, people have started leaning towards that, simply because it's tech and bling.