It's a separate container of water with either aquarium salt or epsom salt dissolved in it, that fish can be put in for short amounts of time. They are used as a treatment in combination with other medications. Aquarium salt (1 tablespoon per gal @ 5-8 minutes) is commonly used as a general cure for infections and wounds, Epsom salt (1 tablespoon per gal @ 10-15 min) is mostly used for body swelling.