All doable and feasible, but ask yourself: do you want to trust your fish to your programming skills?
Haha my girlfriend wasn't even crazy about my idea to timer PWM our aquariums LED light
Because this doesn't control anything, rather tells you information, in my eyes you are still better of with this thing rather than not knowing this info at all.
I would think that part of the project would involve a calibration phase when you were using the Arduino system AND manual pH test and thermometer, and tweaking the software to fully model the parameters - over a few weeks.I would also have a sensitivity setting in the software or control hardware where you could adjust the alarm-limits. I would set the limits pretty tight to begin with so you can see the red light activate without endangering the cute pets' lives.I imagine a further phase where you would put some fishtank water into a separate container and add some NaHCO3, boiling water or whatever and test the alarm points at lower sensitivity levels.