Aquaculture is growing every year by 10%. Due to the fact that the oceans are overfished, the world population explodes and water and protein resources are becoming rapidly rare, the fish production in RAS opens up new perspectives. Recirculation systems can also be installed in existing buildings such as abandoned cow- or pig stables, industrial buildings, warehouses, etc.. The waste energy from biogas or heating plants can be used to heat the indoor fish farms.