diy water level meter
A simple float switch is great if you just want a single level, but as you need to know lots of different levels, you must take a different approach.
Possibly modify a car petrol tank sender, either resistive or capacitive
Bouncing ultrasonics of the liquid surface and measuring the reflected time. May be able to modify an ultrasonic tape measure to do this.
Use a laser, same idea, but much more difficult.
There are commercial sensors, but they are probably not cheap. Possible use hall effect IC and sense the distance of a magnet on a float?
Personally I would investigate the ultrasonic method, it seems the best approach. Either way the output would be analogue and you would need a micro with suitable inputs. Of course you would need to do a bit of maths to scale the output, if you wanted to show capacity in gallons etc. Depends a lot on the shape of the tank.