Kalkwasser will react with the CO2 in the air and create an insoluble percipitate of calcium carbonate. Kalkwasser isn't particularly soluble so the best way is to mix as described and then put in a sealed container to keep out as much CO2 as possible and dose the clear liquid. If you get some of the sediment that is ok, it is either kalkwasser or calcium carbonate depending.
pH maintence isn't just about throwing some chemicals in the tank. You need to understand what pH is and how to deal with it. Most situations where there is low pH is due to CO2 issues at or near the tank. A CO2 monitor works wonders to help understand this, and give the feedback needed to decide the next steps for dealing with low pH. Just note that this isn't the only source of low pH but one of the predominate ones.