Is there anything portable that could be dropped into different sized and irregularly shaped tanks which could measure the amount of change in liquid height?
For this application we would suggest a LMP307 submersible liquid level sensor combined with a Baroli digital battery powered gauge.
The sensor can be lowered into the bottom of each tank and because it is a hydrostatic pressure sensor it will measure the liquid height in each tank regardless of the size or shape of the tank.
The submersible sensor will be powered by the baroli digital gauge which can display the liquid height in metres of water. If you need different measurement units these can be programmed into the digital gauge during manufacture.