Ken wrote:

I've decided to set up an automatic CO2 system for my 20 gal tank.
I've decided to go with an inexpensive "student" electrode as it has
high durability and sufficient accuracy for the task. Unfortunately
the only pH monitors that I have found are the 2 that dupla sells. Is
this it? Or am I going to have to spend $500 for a scientific grade
meter (which I won't).

Not to cause trouble, but what is your reason for building a CO2 control
system? Leaving the CO2 on 24 hours per day works fine, and lots of people
turn the CO2 on and off with their lights.

Do you have a specific application that requires pinpoint pH control, such
that you are willing to undertake the ongoing probe calibration and
maintenance, and risk pH crash due to probe/system failure?