I don't know. So, if I dose up the alk, it should stay level?
Assuming that you have your CaRx setup to keep up with your demands, yes it should stay at that level.
So take note of what the output pressure from the regulator, bpm, and how many mls/minute of the effluent. I usually measure alk because you'll notice more of a change with alk and you will with CA. If your CaRx isn't keeping up with your alk demands, you can increase the output pressure of the regulator (making bigger bubble therefore injecting more co2) or increase the bpm (if you increase the bpm, you'll have to increase the mls/minute so that your solenoid isn't constantly turning on/off) or increase the mls/minute of the effluent(if you increase the mls/minute, you'll have to increase the bpm or output pressure to keep the ph down in the reactor).
If your reactor is keeping up with the demand (meaning your alk hasn't change over a few days) and your alk is still lower than your desired range, dose and alk supplement to get alk within your desired range.
BTW, what type of media are you using?