I was in the local irrigation supplies outlet, and looking around whilst the connectors I’d wanted were sourced, when I noticed a rather natty water tank float gauge made in Australia. I didn’t ask how much it cost. I know the Zimbabwe version (on the right below) is much cheaper and spares are dead easy to find too.
Water tank float gauges on the local market
Zimbabwe Absurdity 
It Just needs a bell for the outside bottle to hit to tell you the tank is empty or maybe it would be better if it told you when it was full, it would be less annoying? What noise annoys a noisy oyster?
Yes, but if the bell rings and nobody hears it (as will likely be the case) does it make any sound?