Salt is the most effective bath treatment, and is used to eliminate ciliated protozoan parasites (including ich in small fish); also used to curb the absorption of nitrite, and to reduce the osmotic pressure exerted by fresh-water on any hole in the skin or gill.
Potassium permanganate and ionic copper are used to treat similar ailments.
Formalin, formaldehyde gas in water, is used to as a fungicide and treatment against some bacterial infections, ciliated protozoans and flukes.
Synthetic antiparasitics such as Lufenuron, Praziquantel, and Chloramine-T are sometimes used in bath treatments, as are insecticides like Diflubenzuron.
Organic dyes such as malachite green, methylene blue and acriflavine are also used to treat parasites and fungal infections.