First the usual gamut of questions...
Tank size
Water parameters, NH3 NO2 NO3 PH, temp
(need actual test results)
Filtration
Established tank
New tank-mates/decor
Prior/recent history of illness disease
Current illness/disease symptoms/duration
Size/age of fish
Tank-mates
This type of abdominal swelling is usually associated with constipation, internal infection, internal parasites, egg-bound or tumor/growth.
The first three usually are treatable, the latter not so much to not at all.
Constipation: Peas/Epsom salt bath/dips.
Internal infection: Either bacterial/viral, improve water quality, medicate water/food.
Internal parasite(s): Medication such as Clout, etc.
Egg-bound: Attempt to strip the eggs (dangerous if inexperienced) or a shot of oxtocin from a vet.
Tumor/growth:
..................................................................................................................................
Have you observed any abnormal, trailing, stringy and/or whitish/greyish feces?
Severe bloat can lead to vents being forced out, prolapse of anus/bowel, popeye, and dropsy*
Try these remedies in order.
If the fish will eat...
1) Try to feed it a pea.
2) Feed it a pea (or something) with a grain of Epsom salt in it.
3) Try a strong Epsom salt bath/dip.
4) Move on to treatment for bacterial infection.
5) ---------------------------- parasitical ---------.
6)Your on your own...
*Dropsy isn't really a disease in it's own right, but rather a symptom of a advanced internal infection/infestation/disease.