I can't effectly help you, without answering the questions I asked about your water perameters. The tank is very much over crowded which adds not only to the difaculty of keeping the water in good condition but is very stressful to the fish which will inturn compromise their immune system making it easier for opertunistic pathogens to take hold. You should be doing frequent water changes and substrate vacuming. I don't know that salt alone is going to resolve this issue, however I can't recomend a corse of treatment without the information I asked for yesterday.