You can go several routes, create your own monitoring system with existing hardware, or go modular. They do sell just the monitoring system for the tanks, you use your own probes. The system itself is simple. You should have a series of probes at different levels, (that part should be self-explanatory, top probe is full and so on...) Now there is another probe that introduces a negative charge to the liquid in the tank then completing a circuit connection when in contact with another probe. That circuit controls the lights that light up the board in which displays the tank level. This can be done by an arduino, or as mentioned before a common module. The adruino would need some logic written for it and a display panel created. If you go that route you can add some fancy features like, alarms, relays to turn pumps off, automatic discharge valves, bypass valves, anything that is electronically controlled. I hopes this helps and does not lead to confusing you anymore....
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