The simplest and smallest is the sill farm, which mixes a mini-greenhouse with an aquarium:
"Designed for the compact city apartment, this prototype fits into a standard residential window sill. The aquaponic unit features a small greenhouse that sits on the window sill and a suspended fish tank that provides organic nutrients for the plants. In turn, the plant life filters the toxic fish waste. The system optimizes the available natural light, thereby lowering power requirements."
Photo Credit: GrowingCities