A saltwater setup is a little more complex than freshwater. Waste products (ammonia and nitrites) from the fishes metabolism are toxic and need biological filtration to break them down. This can be as simple as undergravel filtration or as complex and expensive as a trickle system. Fish like mullet are hardy and would probably survive in a 4ft tank with a simple undergravel system. Go to a good aquarium and have a look at what they have.
Neil