First choice: calcium reactor AND kalk dripper (keeps the pH balanced this way...)
Second choice (and using it at the moment, as I've run out of Ca reactor media...): Dr. Randy Holmes-Farley's balanced DIY alkalinity, calcium and magnesium supplements. Works great - just a little expensive for a 1200 liter system. The supplements are calcium chloride and magnesium chloride (CP grade) bought from a laboratory supplyer, and baked bicarbonate of soda (koeksoda...) and Epsom salt (Engelse sout, aka. magnesium sulphate...) bought from the local chemist.
Hennie