Did you test the water perimeters? :S
And aquarium salt might only be able to kill betta fish in high concentrations, unless you poured it into the tank while your old fish was still in it. The sudden change in the water could have made him go into shock. Betta fish have a high salt tolerance. I've used aquarium salt on all my fish with no problem at all.
If not, there's the possibility of internal parasites, since internal parasites are extremely hard to diagnose because it's all internal. They can look fine but might be really sick inside. But like all sickness, it effect the fish's behavior, so if you watch your fish closely you are likely to notice that something is wrong.
Like my HM was kinda like that, had no appetite and kinda floating at the surface, not moving unless I moved very close to him. But I studied his poop (as weird as that sounds, but poops says a lot) and it looks a lot like one of my female betta when she had internal parasites, so I've been treating him with some maracyn-two and he's becoming a lot more energetic and his appetite has increased tenfold :)
I'd put him in a smaller container (like a pickle jar, so basically a hospital tank) and watch him for several days (changing the water daily of course). Give him a few salt baths might not hurt either, try and watch for his poop. What I look for mostly is the colour (if it's not the same colour of the food your feeding him, looks opaque white and mucus-like and/or unnaturally stringy, he might just be sick with internal parasites). If his poop is not natural, maracyn-two or tetracycline usually does the job. But don't use these immediately, because medicines do tend to stress fish out. So start out with the gentle daily water changes and salt baths :)
Last edited by BakaMandy; 09-07-2009 at 11:21 PM.