I got to talking to my little brother about it ((there's three computer science degrees between us.)) And we came to the conclusion that planning and layout would be the hardest part of creating an automated system. Finding "plug&play" ph and ppm meters would also be a difficult part of the build. Could do the script in python.
As for a Co2 sensor for the tent((I'm going to be running an enclosed tent, everything but the plant, light, fans and extractor are external)) you could probably rig a household Co2 alarm to open a vent on the tent or whatnot if the ppm inside got too high, Instead of setting off an alarm...
...in my head, you'd disconnect the contacts from the piezo in the Co2 alarm, and run them through a transformer and control circuit that would open your external vent if you got above a certain ppm, and would close after an allotted amount of time.