Salt Baths and Osmotic Shock Therapy

Salt baths exploit the osmotic gradient between fish body fluids and the surrounding water to dislodge parasites and reduce stress. Freshwater fish held in a 1–3% (10–30 g/L) sodium chloride bath for 5–30 minutes experience a shift that many freshwater parasites, notably Ichthyophthirius and Trichodina, cannot tolerate; marine fish transferred abruptly to fresh water (a 0% bath of 3–10 minutes) undergo the reverse shock, which kills marine protozoans such as Cryptocaryon and Amyloodinium while most marine teleosts survive brief exposure. The same gradient explains why salt at 0.5–1% is used continuously in freshwater systems to support osmoregulation during handling, transport, or nitrite exposure: elevated chloride competitively blocks nitrite uptake through the gills, directly reducing methemoglobinemia risk. Key rules: dissolve salt fully and aerate, match bath temperature and pH to the source water, and never exceed the fish's tolerance, checked first on a small sample group. Species vary widely; salmonids tolerate 3% short baths, while many cyprinids and larvae are far more sensitive. After any osmotic shock treatment, fish must be returned to system water gradually if salinity differs by more than 5 g/L to avoid killing osmotic stress. Bath water should be discharged away from culture units, since surviving parasites re-entering the system negate the treatment.