running a kalkstirrer

How do you run your kalk reactor? I have been making a slurrey with about 1/2 cup of powder. This seems to last about 3 weeks, then my PH starts to drop. What do you do at that point? Add more, or clean out the stuff that's left and start again with another 1/2 cup? Or if you start with a larger quantity, does it last longer?

I haven't yet figured out the best method...