Licensed medicines available in the U.K. for treating Atlantic salmon infested with sea lice, dichlorvos, azamethiphos, and hydrogen peroxide, can only be administered by bath application. Adverse reactions have been reported to bath treatments including mortalities, inappetance, reduction in growth and reduced louse sensitivity to dichlorvos. The physical constraints of bath treatments are examined and improvements recommended. Oxygen saturation was adequate during treatments but declined rapidly when the tarpaulin was removed. A chemical marker dispersed uniformally both horizontally and vertically in a cage within 5 mins of dispensing indicating dispersal of a medicine is rapid and adequate during treatment. The range in enclosed volumes in 86 treatments was 46 to 146% of theoretical, suggesting potential toxicity due to high concentrations at low volumes and the risk of reduced sensitivity at high volumes. Residual concentrations of hydrogen peroxide varied from 50 to 400 ppm from 5 to 15 mins after the tarpaulin was removed. Water exchange should be encouraged by aerating the cage and flushing at the end of treatment.