Overcrowding is having too many occupants in your saltwater aquarium.
It can have several stressful effects.
An immediate physical and obvious effect is associated with lack of personal space.
Too many inhabitants leads to too much wastes. This leads to pollution. Which is one of our saltwater aquarium emergencies.
Pollution then again goes and leads back to stress, and so on. It is like one big cycle. So try to avoid it!