I found this article online:
This "wasting disease" you are describing is probably pescine tuberculosis. From what I've seen and heard it seems to be most common in livebearers. Unfortunately there is nothing you can do for a fish with TB--it is 100% fatal. Normally I hate advising euthanasia in any case due to fish's capacity to recover from disease, but in cases of TB it is really better to euthanise to prevent the fish from suffering. Furthermore this disease can in rare cases be contagious to humans. People who catch it have large wounds in their hand and perhaps a weak immune system. The disease won't kill a human but it causes a bad skin infection. So, be careful when handling fish and tanks infected with TB. Another thing you should know is that I have heard that bleach does not kill the TB germ. So...anything that comes into contact with these sick fish needs to be chucked.
It is good that you isolate these fish immediately. The disease spreads three main ways--the fry of an infected fish may have the disease passed on, by eating the body of a dead fish infected wiht the disease, or by eating their poo.(:P) So, the other fish in the tank may be perfectly fine. I lost an entire tank to tuberculosis not too long ago...except for my two little Otos who watched all the "hardier" livebearers drop off one by one and never batted an eye through the whole thing.
One more thing to know...the disease can have an incubation period of up to 6 months. So, any fish exposed to an infected fish should be quarintined for 6 months before you put them with other fish or put other fish with them.
Just for the reference, besides extreme weight loss and wasting away some other symptoms which may or may not occur are:
-initial bloating, followed by the weight loss
-loss of apetite, either partial or complete
-curved/deformed spine; hunched over
-inactivity--sitting on bottom or hanging at surface
-symptoms of finrot
-symptoms of dropsy (bloating, scales protruding, and/or popeye)
-long feces hanging from them
-caved in stomach
-alternating seeming extremely sick and then acting ok bloating
-very sudden weight loss
-may either slowly waste away...or else all your fish will die suddenly for no apparent reason
As you can see this disease can mimick many other diseases so it can be difficult to diagnose. Note that these symptoms are not necessarily all present in each individual case of TB...they are just possible symptoms.
Though TB is rare it is important for every fishkeeper to know about it, which is why I have gone into so much detail here. It is essential to act quickly if TB is suspected in order to save any other fish in the tank. And also, avoid pet stores like the plague that have tanks with lots of dead fish (since that is how the disease is spread), or with fish exhibiting any of these symptoms.
Here's another link: