Hi Mary,
Like jdieck said, you don't have to dose kalk if you are using a two part system. Are you having any difficulty with your 2 part system?
I use kalk in my top off for my 120G in addition to my calcium reactor to keep my pH elevated since calcium reactor can depress the pH. I find dosing with kalk cheaper for a large tank than dosing with a 2 part. Especially now that we can get Mississippi Lime Company's food grade lime so cheaply (about $10 for 50 lbs.)
I used to use a 2 part system for my 15G, but being close to my gas stove in the kitchen (which generates a lot of CO2 when I cook) I had a hard time keeping my pH up. I use kalk in the top off there now and supplement alk only if I need it.
Tomoko