you should definately get a filter, it will provide essential circulation for a healthy tank, i would reccomend a power filter like an aquaclear, penguin or for the best performance, an emperor, these filters provide mecanical filtration to remove debris, chemical filtration to remove chemicals and keep the water clear and biological filtration to break down harmful ammonia and nitrite, a filter is an absolutely essential piece of equipment for any aquarium, especially a salt water aquarium
good luck