You really love your fish to spoil them so much 
Its all possible with the Arduino. I also agree that I don't know so much about pH and if you could do it, you might need some expensive transducer. As for the rest start by building 1 feature and coding it as a function that gets called in loop()
Then add the next feature and incorporate it into its own function. The modular approach or stepwise refinement where you start simple and get progressively more complex as you go along....