Hmm...could be swim bladder. Bloated bellies in fish are most commonly a bacterial infection, dropsy, or swim bladder. If the scales of the fish are sticking out, it's dropsy. If your fish is upside down, having trouble swimming (meaning he's fighting to stay down in the water so as to not float to the top), it's swim bladder. If your fish just has a big stomach, no swimming difficulties, and is refusing to eat, it's an internal bacterial infection. For all of these problems, you should move the fish to a QT, lower the salinity to 1.013 over the course of a few days (this is recommended because it reduces the stress level of the fish), and dose with Maracyn-Two for seven to ten days. Make sure to use double the amount recommended on the package for each day (i.e. for the first day, they say add two tablets. Add four instead. On the second day through the end of the treatment, they say add one. Add two instead). They don't make this medicine strong enough for saltwater fish, hence the doubling of the amount. Also, before each dosing, do a small water change because organics in the water can often build up and reduce the effectiveness of the antibiotic.
If you can't find the Saltwater version of Maracyn-Two, go ahead and use the freshwater version. The only difference is the addition of B vitamins to the saltwater one that isn't completely necessary.
Hope this helps!
-lefty