water that is neither fresh nor saltwater, but is somewhere in between. In nature this occurs at the mouths of rivers and swamps near the sea.

brackish water should have enough salt added to reach a specific gravity in the range of 1.002 to 1.008. Since brackish water is generally a mixture of freshwater and saltwater, a high quality saltwater mix works best but pure rock salt will work in a pinch.
NEVER USE IODIZED SALT!
A hydrometer is needed to measure salinity but be sure that the one used will measure the necessary range. Most hydrometers found in Aquarium stores are made for measuring pure saltwater so they will not measure the lower salinity needed for brackish. A couple direct reading of specific gravity and ppt. that will work for these lower levels are: Instant Ocean Salinity hydrometer manufactured by Marineland Labs, and SeaTest Full Range specific gravity Meter manufactured by Aquarium Systems.

Some fish live in salt water but are spawned in brackish or fresh water and vice versa. There are several brackish species available in the Aquarium hobby, see freshwater Fish for descriptions.