The invention relates to a submerged observation unit (6) for a fish tank, wherein the observation unit (6) is suspended in a node cable (5) from a hoist winch (3) of a surface buoy (2). The node cable (5) is torsion-resistant, wherein the node cable (5) is at its upper end wound on the hoist winch (3) having a horizontal drum axis. The observation unit (6) is arranged to be motorized and to azimuthally rotate about a vertical axis. The surface buoy (2) is arranged to be motorized and to move along a main span (10) of a surface cable (1), wherein the main span (10) is arranged to be spanned across a float ring (9) of the fish tank.