Hi,
We actually sell a tank level sensor that has a range of 5 meters here:
As long as your tank is less than 5 meters tall this would be exactly what you are describing. However I fully understand this is a much more expensive solution than you were probably looking for. This device is more for our industrial user base. It sounds like you might fit more in our Doc Brown user base 
This however is nothing more than an ultra sonic sensor device like you’re describing, just more refined than a standard Arduino based device. My recommendation would be to source an I2C ultrasonic distance sensor(just google that and you’ll find them). This can be connected to the I2C port on the MCP23008 board, however be careful to ensure that the I2C ultrasonic sensor is 5VDC tolerant on the SCL and SDA lines used for the I2C data communication. You should be able to source a 3rd party Arduino library for the sensor online, if you can find a Particle library that would be even better but honestly porting an Arduino library to Particle is cake.
Hopefully this helps.