The fish part is just like fresh. Some fish get along and others don't. Then you add the crabs and shrimp. And snails. Even some snails will kill each other. Then you have hitchhikers like bristle worms. And once you get to corals that's an entirely different level.
So complexity can be no more complex than freshwater if you want that kind of tank.
So complexity can be no more complex than freshwater if you want that kind of tank.