How long have you had these fish? If you have had them for a long time and they are now suddenly dying, there must be a problem with your water or maintenance. If they are fairly new, this could be a problem that they had before you purchased them. You should check with your vendor and see if anyone else is having a similar problem with fish purchased from them at the same time. Also, is there any chance that something other than water may have been added to the tank recently? Sores on the body could be bacterial or they could be from external parasites I believe. Did you quarantine these fish after you got them, and if so, for how long? There are so many possible reasons for these fish deaths, that it may be hard to pin down. How are your other fish behaving?