Firemouth wasting away but still alive

I don't know how, but my firemouth is still hanging in there. I haven't seen him actively eat anything in months. Unlike the other fish, he hasn't grown at all since I introduced him. I'm guessing he has internal parasites but it's impossible to say for sure.

The other fish are doing swimmingly, so to speak. The oscars occasionally exhibit mating behavior and have picked out a nest. The severums are huge, nearly the size of the oscars.

I salted the water pretty heavily a month and a half back, an effort to help the firemouth and stem anything the other fish may have picked up from the afflicted one (I also suspected gill flukes as all of my fish "flash" -- bashing their sides against the sand and objects).

My Rena XP4 cannister filter developed a line of salt creep --evaporated salt worming its way out of the cannister like one of those ash coal warms during the fourth of July. Through extensive and repeat water changes I've dropped the salinity back to normal, though it's still fairly brackish. One more 50 percent water change should drop it back under 1.002 in salinity range.