Fish pen (1) having a double net wall (14, 16, 20, 22) wherein a lower part of the net walls is attached to a pulldown body (36) with an upper attachment point (28) and a lower attachment point (30) with a certain distance to each other, and wherein, by pulling down the pull-down body (36), the net walls are tensioned such that they are spread out from each other, and are not in contact with each other.
