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.
An estuary is the part of a river where it meets the sea. Typically, estuarine waters are slow and sluggish, and often very silty and fertile. As a result they are not always as attractive to look at as the clear waters of a mountain stream, but they are tremendously productive. This apparent contradiction is because relatively few fish and invertebrates can tolerate the fluctuations in salinity. On the other hand, those animals that can live there do so in enormous numbers.