Back when I started my project, I did a lot of research on DIY PH and EC probes and circuits and learned that these circuits are very sensitive to electric noise. This had me greatly concerned as I’m just a newb hobbyist and had great doubt I would ever be able to isolate the circuitry well enough to get viable readings from any of the probes. I presume you’ve also encountered similar information in your development. That said, consider what Whitebox Labs did on their Tentacle and Tentacle Mini shield to galvanically isolate their circuits and try to reverse engineer. Having little to no confidence in my skills, I am definitely gonna save up for the shields, and circuits and probes. I saw THIS youtube video of a guy that used them for his reef/aquarium.
Regarding the $15 Mega w/ Ethernet Shield, I found the pair on fleabay for $30 from China and purchased them, but I’m seeing mixed reviews about extremely cheap Chinese tech, and when considering the gravity of what we’re doing this for, that’s one area I didn’t feel comfortable entrusting my grow to the cheapest one I could find. Don’t get me wrong, I know the Chinese name is catching a bad rep regarding lower quality craftsmanship and engineering, but when comparing to American made prices, the better savings is with the cheap Chinese products, that is until I get rolling and producing. Then of course I can shop for higher end gear. If I didn’t go with the Freetronics Ethermega, I likely would have got an Italian made Arduino.