Where the ocean meets outward flowing rivers, saltwater and freshwater mix. These areas are called estuaries, mangrove swamps, and salt marshes. The water, which is part saline and part fresh, is referred to as brackish. These brackish waters are home to an amazingly diverse and unique group of animals, some of which are commonly available to keep in the home aquarium. These fish include archers, scats, monos, certain livebearers, gobies, and others.
Brackish water aquariums, as mentioned above, are unique. They are usually conversation pieces and allow your aquarium to be different than most others. They also, despite some myths, are easy to maintain since the fish from brackish waters are designed to withstand frequent salinity and water parameter changes unlike both fresh and saltwater fish.
Fish that live in these areas are able to tolerate a wide range of salinities. Brackish fish do best in water with a salinity of 1.015, or 1/4 cup marine salt per gallon, and a pH of 7.5 to 8.4.