Bio-filter unit (1) intended for a fish farming system, said bio-filter unit (1) comprising a container (2) with a filter media (3) and an inner architecture, said inner architecture comprising at least one vertical tube (5) having an open input end (6) and an open output end (7), wherein the bio-filter unit (1) is connected to a pump (8) enabling pumping liquid and filter media (3) from a bottom (9) of the container (2) against a top (10) of the container (2) through said at least one vertical tube (5), wherein an upper plate (15) is arranged inclined downwards from and around the open output end (7) of the at least one vertical tube (5) and ends at a distance from a wall (13) of the container, wherein the upper plate (15) Is perforated and wherein a regeneration water outflow (16) is arranged under the upper plate (15).