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Re: CO2 question

Max, with respect to the pH controller, they exist, but my experience with
industrial chemistry tells me that they have to be checked regularly.  I
had one online once (not in an aquarium), and I had a lot of trouble with
contamination, but the system was nasty, with feeds lines of wildly
varying pHs 1-14, at the electrode we were aiming at a pH of 6.5. We
checked it three times a day, every day and it had to be serviced on a
weekly/monthly basis.

The electrodes can become contaminated and don't last forever.
Combination type electrodes are the least expensive, of the ones that I
have priced, at less than $100.Can or cheaper.  Some are enclosed so that 
you don't need filling solution or maintenance, and they last for up to one

All pH electrodes that I know of, with a delicate glass membrane at the
sensing end, need to be in water (or hydrated) to work. The glass membrane
becomes hydrated, usually after a couple hours soaking, and are ready to
read. So yes, they can stay in water forever.  Some pH meter's drift
also, the electronic part of it, so they need to be calibrated regularly.
In an industrial setting, usually before every reading, or at least once a
day. In addition to all that, the pH electrodes are temperature sensitive,
small water temperature variations will affect the pH reading.

What I have seen recommended here, and makes a lot of sense, is to just
feed CO2 slowly, and not try to control pH using a pH controller.  What I
suggest, if you really want pH, just set up a pH meter and monitor the pH
for a long time ( a year or so). If your system is reliable, and you have
worked out the bugs of the pH system, then automate it.