The present invention relates to a system comprising a container (102) being partly submerged in a fish farm and filled with a softened brackish water having: a salinity in the range from 0.5 g/kg to 15 g/kg, determined as the total mass of Na+, K+, Ca2+, Mg2+, Cl-, HCO3-, CO32-, and SO42- ions being present in a sample of one kg of the water, and a Ca2+ content of ≤ 100 mg/kg, determined as the mass of Ca2+ ions in a sample of one kg of the water, an oxygen supply (101) adapted to adding oxygen to the softened brackish water, and a distribution manifold (107) for injecting softened brackish water to the container.