I have a 25,000 gallon inground block cistern. I have wrestled around with this more than once with mixed results. The best pass I made was using a beach ball attached to a pvc pipe inside a pcv pipe. When the inside stick got down to the top of the outside stick the wire attached to the inside stick would clang the bell on the outside of the house. Lots of issues kept coming up that I finally decided I did not have the time to mess with any longer and it would be just easier to look in the cistern once in a while. Shortly before I moved on to something else I found the water level system they use in Alaska to keep everybody's cistern full. They use an in-tank sensor wired to an outside digital level gage. The gage has a light option you can buy that will light up when its below a certian level. The in-tank unit is made of stainless steel and extends down from the top to nearly the bottom and the wire routes into the house. Seems like the price was $300 for everything and would not take 30 minutes to install and setup. I was told in Alaska where these are used the water guy drives around town and checks the remote gage to see if you need water and adds whats needed. Serach on water gages or water levels using Google and you should find the mfg, seems like they are on the East Coast somwhere. Good luck, ric