What's wrong with my angel?

I have two zebra angel fish, a couple years old. The smaller one has
a bulge in the middle. It looks almost like it swallowed a marble. It's
color is good, it's eating, but the bulge seems to be getting bigger, especially
on one side. The other angel's stripes are very pale, but it is eating
good too. Does anyone know what could be wrong?

My water tests are all good. I never tried to upload a picture
before so I don't know if it worked.