We get someone with this problem on here every day or every other day. I always ask if it's necessary to have the Arduino Uno/Nano etc. in the first place. The ESP8266 is way more powerful than an Arduino UNO/Nano and in nearly all cases, all tasks that need to be done can be done by the ESP8266 itself. For instance, if you want to read a pH sensor (which your sketch doesn't do at present, but let's put that aside for now), you can easily do that directly from the ESP8266. This saves you the trouble of having to connect the ESP8266 to an Arduino.
So short recommendation: ditch the Arduino and continue with just the ESP8266.