Yes, they do need land. And brackish water. I kept two fiddlers for 3 years (I started neglecting them for awhile) in a tank with just topsoil mud and a brackish water pool. They liked to bury themselves and make mud balls sifting through the mud for food. They were fed freeze dried brine shrimp.
IMO fiddlers won't last longer than a year without access to land. And all the while they'll be suffering.