reef-pi provides http json API. Entire UI is powered by the API. API requests needs to be authenticated using session (cookie). He is an example of getting the list of equipment

curl -X POST -c cookie.txt -d '{"user":"reef-pi", "password":"reef-pi"}' http://reef-pi.local/auth/signin
curl -b cookie.txt http://reef-pi.local/api/equipment

Every reef-pi installation ships with its API doc, linked directly from the footer section of the app. A copy of verion 3.x API is accessible from here