The invention relates to a lice treatment system for fish, comprising: at least an inlet (1), at least a first separator (2), at least a pipeline (3), at least a second separator (4), at least an outlet (5) and at least a primary heating circuit (7), where the first separator (4) is arranged after the inlet (1) to separate fish and water, where the pipeline (3) extends from the inlet (1) to the outlet (5), where at least a part of the pipeline (3) forms a liquid bath for the fish and is designed for, in processing terms, closed circulation of a treatment liquid, and where the second separator (4) is arranged to separate fish and treatment liquid so that the fish are passed to the outlet (5) for release of treated fish and the treatment water is led to the primary heating circuit (7) that extends back from the second separator (4) to the part of the pipeline (3) forming a liquid bath for the fish. According to the invention, the system comprises a separate outlet (10) from the liquid bath which leads to a secondary, in processing terms, closed heating circuit (9) that is designed to discharge a part of the treatment liquid in the liquid bath and replace it with treatment liquid that has a higher temperature than the treatment liquid that is discharged.
