Interesting, I am going to bookmark this thread. I have begun working on the design of a more simple system based on an inexpensive PLC. I have both a planted freshwater tank and a reef tank. My goal is to automate several of the functions on the saltwater tank but I am starting with the simpler freshwater tank. I have a temperature probe and it now reports temperature. I have a pH prob and it reports pH levels. I can have the PLC turn on and off the heater using the temperature input and can turn on and off the CO2 based on the pH level. And the PLC turns on and off the lights according to time of day. I am planning to establish a water top off this weekend as I just received two micro level sensor which can detect water level. The above is easy and relatively inexpensive.
Next, I want to measure ammonia and CO2 directly and looking for sensors for these. I am also going to work on a semi-auto water change system, (mostly in view of the larger saltwater tank not the smaller and easier freshwater tank.) Going to put two water barrels in the garage and run plumbing into where the tanks are. Eventually, I will push a button to empty a certain amount of water into a waste barrel, then, push a second button to move a pre conditioned amount of water into the tank from the clean barrel.