The present invention describes a filter intended for separation of a sludge/solids fraction of a water flow, wherein the pre-filter comprises a sludge/solids filtering unit and a sludge/solids removing unit, and wherein the filter is a pre-filter positioned as a first filter in a filter system in a fish farming system. Furthermore, the present invention is also directed to a fish farming system comprising a filter according to the present invention, and also a dewatering unit intended for dewatering the sludge/solids fraction. Moreover, the present invention also refers to a method for separation of a sludge/solids fraction of a water flow in a fish farming system, said method comprising directing an outlet water flow from one or more fish tanks to a pre-filter according to the present invention； and separating off a sludge/solids fraction and directing this fraction to a dewatering unit.
