I don't think you control them as much as just monitor them. We just did a slew of them on a data center project. I think by default they have an RS-232 port that is set up for modem dialing, you could probably find the protocol and just ignore the dialing strings.?
Or you can get a gateway that converts the RS-232 port to Modbus IP or BACnet IP. Our vendor used the Modbus IP gateway to connect to an electrical power monitoring system. With a BACnet/IP gateway you could probably just use the BACnet/IP functionality in a 3-series processor - free for 50 points but spend for that 51st point.
Usually there aren't a lot of parameters you need from this - tank level, tank low limit, tank high limit, tank overfill limit, maybe tank temperature and temp high limit. With BACnet you can just pick and choose which points to grab.
