Device for treatment of fish in a water mass for different conditions, comprising: a housing defining a channel with a treatment chamber suitable to accommodate one specimen of the fish, and the housing comprises treatment members for treatment of the fish in the treatment chamber, wherein the housing comprises means adjacent the channel and which is adapted to be switched between a starting position and a treatment position where the residence volume of the treatment chamber is reduced, characterized in that the channel is tubular and comprises: - an inlet connected to an intermediate chamber and pre-chamber connected to a narrowing channel connected to the treatment chamber connected to an outlet, where the inlet and pre-chamber is configured to be able to contain a plurality of fish, while the treatment chamber is configured to be able to contain a single fish at a time, - flaps (67, 69, 70, 71, 73) set across the longitudinal direction of the channel 38, that is adapted to be switched between an open position where fish can pass through and a closed position, where flaps (67, 70) arranged in the intermediate chamber, the pre-chamber and in the treatment chamber are adapted to be moved axially to move fish forward when in a closed position, and the means for reducing the residence volume of the treatment chamber comprises curved plate bodies in connection with the outer wall of the channel, and which are adapted to be pulled/pushed outwards and inwards to be switched between said starting position and treatment position, which plate bodies comprises: - two mutually sideway arranged plate bodies (52,54) which are adapted to be pushed towards and from each other, and the bodies are curved so that their inner side is facing each other and wherein the curvature is adapted to a normally curved shape of fish in longitudinal direction, - upper and lower plate bodies (53,55) curved correspondingly to the longitudinal backside, and respectively belly side of the fish, and with the inner side of the bodies facing each other, wherein - the device further comprises a plurality of scanners for scanning and mapping the surface of fish in the treatment chamber, and - each treatment member comprises a mouth-shaped head mounted to the end of a tube which is connected via a drain line to a source of negative pressure, for the removal of parasites on the fish, and - a data unit configured to store the surface condition of the fish and to control the treatment members. - an inlet connected to an intermediate chamber and pre-chamber connected to a narrowing channel connected to the treatment chamber connected to an outlet, where the inlet and pre-chamber is configured to be able to contain a plurality of fish, while the treatment chamber is configured to be able to contain a single fish at a time, - flaps (67, 69, 70, 71, 73) set across the longitudinal direction of the channel 38, that is adapted to be switched between an open position where fish can pass through and a closed position, where flaps (67, 70) arranged in the intermediate chamber, the pre-chamber and in the treatment chamber are adapted to be moved axially to move fish forward when in a closed position, and - two mutually sideway arranged plate bodies (52,54) which are adapted to be pushed towards and from each other, and the bodies are curved so that their inner side is facing each other and wherein the curvature is adapted to a normally curved shape of fish in longitudinal direction, - upper and lower plate bodies (53,55) curved correspondingly to the longitudinal backside, and respectively belly side of the fish, and with the inner side of the bodies facing each other, wherein - the device further comprises a plurality of scanners for scanning and mapping the surface of fish in the treatment chamber, and - each treatment member comprises a mouth-shaped head mounted to the end of a tube which is connected via a drain line to a source of negative pressure, for the removal of parasites on the fish, and - a data unit configured to store the surface condition of the fish and to control the treatment members.