There seems to be a few things going on here. The water is foamy, have you added medication or something? If not, you'll need to figure out where that is coming from, and probably do some water changes if it is organic in origin.
Feeding peas is an old fancy goldfish treatment for constipation, it won't help here. Likewise, Epsom salts is a freshwater treatment, it won't help in marine tanks. Think about it - magnesium sulfate is the fourth most common salt in seawater, adding a bit more won't change much at all.
Unfortunately, I think this fish has edema, fluid in its tissue. The two primary causes for this are liver or kidney failure. Like in humans, there is no cure for this. The reason I think this is that the swelling seems throughout the belly. If it were constipation, the swelling would be more localized.
Sorry I couldn't offer a treatment.....
Jay