i2c is going to be a big part of this project.

Each device will be have the means to monitor a tank; a pH sensor, temperature sensors, relays to control lights, heat and pumps, an lcd screen for displaying tank status, and anew arduino nano to coordinate it all.

In the instance of the master unit it will also have a Pi Zero to host the Webserver and request values from the Nano via i2c to display on that tank's page.

One thing I would love to do is daisy chain several slave devices to one 'master' unit; the Raspberry Pi in the master unit could request status updates from each of the attached slave Nanos for display whenever that tank's Web page is loaded. the Pi could also send commands to the slaves to put ad hoc messages on the screen, or update the thresholds for the monitoring of pH and temperature (thresholds being stored on the Nano's Eprom).

One thing I would like to explore is using a 'software i2c' on the arduino. this would allow the arduino to control the lcd display on its own i2c bus and leave the main, 'proper' i2c for Pi/Arduino communication.