Think we might have it, dropsy
I'll type a bit of it for you to see what you think
Caused By
Bacterial and/or viral infections, also metabolic or nutritional disorders.
Obvious Symptoms
Markedly swollen belly, protruding scales, reddening at vent or base of fins, ulcers on body, and long, pale faecal casts. Affected fish may go off their food and show darkening of colouration, pale gills and a 'pop-eyed' appearance. Fluid may accumulate in the body cavity, with discolouration of internal organs.
The treatment bit is a bit long winded but i'll give you the lowdown
check water conditions, make sure they are ok, isolate fish, feed only best possible food. If symptoms do not improve treat with a broad spectrum antibiotic or similar.
I've just copied from a book here so the credit for this goes to the writers of the Interpet Manual of Fish Health for this one.
Again good luck
Rich