Water level

Aluminum strip on the other side of the resistors.

Water level

Here you see the resistors, in a serial configuration.

Resistors on a plastic tube

Water level

To sense the water level I needed something solid state, durable and easy to make. The above circuit is the design I'm going to use. It consist out of 10 resistors of 1k in a serial configuration and one of 10k to limit current. It will be connected to 5v in order not to harm any fish. How it works? The resistors of 1k will be mounted on a plastic tube and a aluminum strip will be glued to the other side. When the water level is very high all the resistors will be shorted and no current will go the the Arduino. When the water level is very low all the 1k resistors will be active, in total 10k, so current will go to the Arduino. When the water level is somewhere in between some of the resistors will be shorted and some will not be shorted, resulting in a different current.