I am new to the forums, but I have been searching obsessively to find the answer to this same question. My fish have this SAME disease I have been wracking my brain to figure out what it is. Just like the OP said, my fish have slight bloating in the front, but then their rear is really droopy and wastes way. Color and appetite stay normal until the end, but in the very last stages (right before death) the wasting becomes so pronounced that the fish have difficulty swimming. Some fish will last for up to a month while others will be dead within a week, and it seems like whenever one fish is moved to quarantine another one has become sick.
Some info about my tank, it is 16 gallons up and running for six months now, heavily planted, with three guppies, four platys, some tiny fry, and an unknown number of cherry shrimp and MTS (recently added four assassin snails and the problem began about two months ago). Temperature stays between 73 and 74, Ph is a little bit above 7.2.
I am convinced that it must be something internal, as if it were environmental, it would affect more than one fish at once (imo). I am really worried now that it is fish TB, so I have been trying to remove fish that are showing symptoms as soon as possible, but as my tank is cooler than most tropical tanks, is this even possible? I've heard that Mycobacterium marinum's ideal temperature is closer to 30 degrees celsius. I've observed poop and it is definitely NOT white and string, it looks the same as it always has. Anyway, I will try some of the medications that previous posters have suggested, but THIS IS SO FRUSTRATING!