The present invention relates to a method and system for treating fish for calcium sensitive ectoparasites by containing the fish for a period in softened seawater having a salinity in the range from 0.5 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 of one 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 kg of the softened seawater.