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.