The Aqua Farm is a building unique to the Aquarion tribe that costs five stars and produces two population. The building is unlocked by the Aquaculture technology and can only be built on ocean tiles with crop (visible once Organization has been researched). Windmills produce one population if placed next to an Aqua Farm.
The Aqua Farm was originally added in the Path of the Ocean update and available to all tribes; it was shortly after removed in the Patch of the Ocean update before being reintroduced as a building unique to Aquarion.