Brackish water is water which contains more sea salts than freshwater but less than the open sea.
Moreover, brackish water environments are also fluctuating environments. The salinity is variable depending on the tide, the amount of freshwater entering from rivers or as rain, and the rate of evaporation. As a result many brackish water fishes are tolerant of changes in salinity, and in fact many positively benefit from similar periodic changes in aquaria. The environments from which brackish water fishes are collected are diverse. Estuaries are the best known.