Incorporate some calcareous material in the filter, and decorate the fish tank liberally with sea shells. The exact salinity varies for the various species. None will thrive in freshwater, but not all require full strength sea water. They will do well in water with a specific gravity of 1.005 to 1.015. The water current should be quite strong but not turbulent.
Mudskippers are territorial when mature.
Both Periophthalmus species are essentially goby-like, but with flattened faces and with the eyes locate right on the top of the head. The dorsal fins are patterned with bright blue and red when mature. They display to one another by raising and lowering this fin. They feed on chunky live foods like bloodworms, tubifex worms and crickets. The eyes are located in a more normal position.