its an ammonia spike it hapened to me as well do regular water changes and DO NOT FEED YOUR FISH add more aeration to the tank
To me, this resembles hemorrhagic septicemia - usually the last step of a series or unfortunate events and ailments. It is the infection of blood and is mostly deadly. Caught in time, there is a roughly 50-50 chance but it is not easy. Erythromycin (phosphate or base dihydrate) could be tried (they will wipe out your filter too). I didn't like that you stated in your 1st thread that LSN's behavior changed when the TSN was introduced - you said he became a picky eater. My guess, it has something to do with it. Perhaps a bacterial infection was introduced.
As soon as I saw the picture I the same as you. looks like Hemmorragic Septicemia which is a bacterial infection of the blood. I do differ as far as treatment goes, i would use Triple sulfa as it is more effective against aerobic gram-negative bacteria while Erythromycin is most effective for gram-positive bacteria where Streptococcus sp. is the suspected cause. Either way it may be too late for this fish, however if you wish to try since the fish is isolated treat with 3X sulfa. Sulfa is not hard on nitrifying bacteria so you do have to worry about whiping out your BB during treatment. GL.
All the medications mentioned in this thread should be available at your LFS or Petsmart.