From The Aquarium Wiki
Brackish is a term used in connection with water and salt. It refers to water in the natural environment which is part way between freshwater river and saltwater ocean (such as estuaries and river mouths). Brackish water has various degrees of salinity (the amount of salt dissolved) depending on how far or close to the sea the environment is (the nearer, the higher). In the aquarium brackish water is maintained by using marine salt (not aquarium salt which is a tonic salt), and a hydrometer or refractometer to measure salt levels (the specific gravity, or S.G.). Typically brackish water has low-end salinity, with a S.G. of 1.001-1.015. Typically this is 1 tsp of salt per 5 litres.
See salt article for more details.
- Badmans Tropical Fish - Starting a Brackish Aquarium