Reactor problems

[Gools]

I can't figure what I'm doing wrong. My reactor has been running for over 3 weeks. The first 3 weeks, everything was great. Alk at a constant 9ish, and calcium at around 400. At the end of October I did a 40g water change, on my 180. And ever since My reactor hasn't been working good. My Alk is going from 9.5 to 8.3 overnight and back up to the low 9's again. I haven't adjusted my bubble count, or effluent. And I can't get my Ph of my efflunt to stay constant. It's bouncing around from 6.2 to 7.0. Anyone have any suggestions? It's extremely fustrating, and wondering if using a 2 part would be easier in the long run. Thanks guys.