Here's a shield designed by Liquiduino, to monitor liquid levels, using sensors (buoys) resistive bridges in order of 0 the 220 Ohms.
Has an input and an output event for interaction. The prototype is as follows:
It also has a RTC and an EEPROM to register for timed events of variations resulting from the measurement of liquids.
It is still a prototype still in experimental phase, regarding the response of several sensors.
Liquiduino Shield for Arduino