Not stupid Ca(OH)2 and as mb said, it's used to raise Ca and alk levels. It comes in powdered form and is mixed with good freshwater (RO filter for example). Then people usually drip it slowly since kalkwasswer solution has a pH of 12 or so.
I was thinking of getting seachem reef advantage calcium to do a drip. The only thing though is that it says the pH is not caustic. Does that mean I wouldn't have to use vinegar and this would be easier to use? Or is it better just to get true kalkwasser and make use of the vinegar?
I tried the vinegar thing and didn't like it. I didn't see much benefit and developed a strange algae growth after I started using it. The algae eventually receeded and disappeared after I stopped using it. <shrug>
I'd recommend topping off all evaporation with regular kalkwasser and just supplementing for what you need beyond that with a good 2-part.
If you have really high Ca/Alk demand and want to automate things there is always the reactor option...