Terrible Advice Tuesdays (T.A.Tues): Saltwater aquariums smell bad
The rest of the story: Can saltwater aquariums smell bad?
Do they normally smell bad?
If your saltwater tank smells bad, the first place I would look for the problem is water quality. A tank with poor water quality (high ammonia or nitrates) will likely smell bad. And a tank with poor water quality probably has something driving the poor water quality such as a lack of chemical filtration (namely activated carbon) or little or no surface skimming.
Even a protein skimmer cup that is full of nasty skimmate should not be noticeable when you are standing in front of the tank.
Simply put, a smelly saltwater tank means something is wrong.