Brackish water fish are species that inhabit the areas where freshwater meets saltwater. This transitional zone, also known as an estuary, is characterized by fluctuating salinity levels that are neither fully freshwater nor fully saltwater. Brackish water fish are adapted to these varying conditions and can be found in different parts of the world, from coastal areas to river deltas. Some popular brackish water fish species for aquariums include the figure-eight puffer, knight goby, monos, scats, and archerfish. When keeping brackish water fish, it is important to maintain stable water parameters and provide appropriate habitat and diet according to the specific needs of each species.