Betta Constipated?

About a week ago I noticed that one of my male Bettas (beautiful
split-tail) was bulging in the gut, looking as though he'd swallowed a
marble. I did the requisite online searches to see what I could do. I
got the strong impression this is a common problem.

His scales remain smooth, not pine-coned, so I think I can rule out
dropsy. His behavior is fine - he's not especially listless or having
difficulties that I can see. No spots or signs of disease. Two other
bettas in the tank with him are quite healthy.

Following suggestions I'd found, I stopped feeding him. After 4-5 days,
this made no visible difference, so I added salt to the tank - 1 Tbsp
per ten gallons. Again no difference. Two days ago I boiled and mashed
a pea and fed it to him (laxative, apparently), and again no result,
except that he's bigger by a pea.

The bulge is now uneven, and certainly no smaller. What have I missed?
What else can I try.

--
Eric Schreiber
www.ericschreiber.com