Reef-Pi was born out of my own requirement to automate daily chores of my reef tank. Commercial controllers were too expensive for me, and I was learning Raspberry Pi and electronics (3 years before). So, I decided to start building my own controller and make it opensource so that anyone else can make it. I also wrote the bulk of the controller software. After three year reef-pi has more than 50 builds (by reef keepers other than me) and across three continents.
What it is
Reef-Pi uses Raspberry Pi as the main computing environment and handful of Adafruit and dfrobot breakout boards to sense reef tank environment,(temperature, ph, water level), and control AC (relay based) & DC (pwm based) equipment for various reef keeping chores (like daily light cycle, dosing chemical reagents to maintain stable ph and Calcium levels, switch on/off heater & fan to control temperature etc).
reef-pi is modular, allowing users to build it for a single/specific purpose (like controlling light) or for an entire range of reef keeping requirements. reef-pi official website provides comprehensive guides to build a reef-pi controller. reef-pi's original build thread is in reef2reef (world's largest reef keeping forum), where daily/weekly progress of the controller development (along with hundreds of images/videos of other's reef-pi build) is documented.