The present invention relates to a method and system for treating fish forcalcium sensitive ectoparasites by containingthe fish for a period in softened seawater having a salinity in the range from0.5 to 15 g/kg, determined as the total mass of Na+, K+,Ca2+, Mg2+, Cl-, HCO3-, CO3 2-, and SO4 2- ions being present in a sample ofone kg of the softened seawater, and a Ca2+ content of<;= 100 mg/kg, determined as the mass of Ca2+ ions in a sample of one kgof the softened seawater.