One possibility is to write a script (probably in Home Assistant?) that can send a command to OpenSprinkler via its HTTP API to trigger actions when the tank level sensor triggers.
Another possibility is to use some sort of remote switch, such that when the tank level sensor triggers, it sends a signal to trigger a relay via RF or any type of wireless signal. There are off-the-shelf wireless rain sensors that work this way. I would think probably some sort of wireless tank level sensor also exists.
How is your tank level sensor integrated with HA?