Hi Raul
If you don’t have experience on Arduino IDE for Esp8266 , you should follow the installation instructions of GitHub.com/esp8266/Arduino.
If you find difficulties may be the best will be that you tell me your configuration and the type components you want to use and I will compile and build the firmware for you. You will have only to flash the firmware and the config.txt file on the Esp8266 .
Regarding Raspberry Pi it is possible to Port my code to it but it is going to be a long difficult work. I strongly suggest you buy another Esp8266 that will cost only few dollars! Or if is possible to use one of the OpenSprinkler IO to drive your relay pump, then you may use a single Esp8266 connected to the Ultrasonic sensor on tank 1.
Today there is no possible connection of an Ultrasonic sensor to a OpenSprinkler unit!
Keep in touch!
Paolo