Also, its possible you can get digital I2C sensors for a lot of these devices for stuff like the Temperature and pH (And maybe Level, I don't know how you intend to do level). These only require 2 wires for all of them, but you will need them to be addressable up to at least 8 devices. Another question is how quickly do things need to react? If you are talking about cycling through measurements every 100+ms, then you are fine, but if you need it to react fast to something, then the arduino is not a good fit, since most likely you will be unable to perform 'real-time' adjustments. That being said, you are working in liquid tanks, which tend to take seconds to react to any disturbance at all.