Water/Solution Level in Tank

I’ve been considering trying to figure out a Raspberry Pi simply so I can automate my outdoor garden. I realize that using the Grow Hub might be like using a 747 to transport a single person in this regard, but it would be worth it if it could determine the level in my solution tanks and let me know when I need to add water… or even better, turn on one of the outlets so that I can use a solenoid valve to do the filling for me.

(With my garden being outdoor and connected to my rainbarrel there are lots of variables in play and it would be great if I could get not only the valve to turn on, but also determine the water/solution consumption through the season.)