Thanks Crispin, having it sleep would be interesting. I really wanted to get away from having a device at every tank to manage though. There was a company that was working on a “puck” that you would throw in your tank and it would measure your temp, ph, etc. There is pretty big market for that and i would love to figure that out. There are over 68,000 towers in homes around the world and the Europe market is set to open next year.
Depends on the distance between tanks. I. Think individually is a better (redundancy :))
Puk is a nice idea. If I DIY it then I would add a barometer. Nominate one as a reference and the diff between ref and each thank times area is volume 
Barometer from the memory you had a cheaper sensor with cm accuracy (less than than 1kpa) I might be wrong though.
Openhab can certainly show MQTT streams, but for data logging, you might want to check out another open source project: Thingsboard. (www.thingsboard.io). We have had a lot of success with 18b20s being read by a NodeMCU, and the data being transmitted via wifi to the thingsboard server.