Need more information. What are your exact parameters? How many fish do you have in there now? How did you cycle the tank? Do you have live rock, filter, sump/refugium? Did you quarantine the fish? Do you have a QT? How did you acclimate? Really need a lot more detail.
Right off the bat I'm going to say you have the answer already. Overstocking. Plus damsels are bullies and are not the best tank mates. Overstocking leads to stress and eventually disease. I would do some more research on sw
tanks to help you with a good clean healthy tank. There are lots of great articles on this site alone.