An apparatus (1) for injuring or killing undesired organisms in water (9) in a channel (2). The apparatus (1) comprises an insert (4) which is arranged to be releasably attached to the channel (2) on the inside (20) of the channel (2), a plurality of electrodes (3) that are attached to the insert (4), the insert (4) being formed from an electrically insulating material and the plurality of electrodes (3) being connected to a power-supply unit. A method for using the apparatus (1) in the channel (2) is described as well.
