Hi and thank you for sharing the very useful project :) 
Question: If I need to scale it up to monitor 5 tanks, what should be done? I guess that I would have to 1) connect each sensor to separate GPIO pins, 2) modify in some way the database and the webGUI code, 3) Create 5 cronjobs. Can you give an example of this project can be used to control multiple tanks? Thank you :)