Put some Kalk in your makeup water until you can afford the reactor setup.
The key to kalk in your ATO is to only have the ATO on at night, and to make sure your CA levels are perfect before you start. Kalk will hold those levels there. The reason I say to have it on only at night is because it will raise your PH, but at night your PH is lower (usually) and this will bring that up through the typically low PH time. Also, make sure the PH of the makeup water for your ATO is 12 after adding kalk.

If I am wrong, someone please tell me! This is how I have been doing it for several months now.