This article is from the Aquaria: Disease, Algae and Snails FAQ, by Elaine Thompson, Thomas Sasala and George Booth
Fish swells up like a balloon and may show popeyes. It may recover
with no treatment and may die despite it. The swelling is because the
fish is absorbing water faster than it can eliminate it, and it can be
caused by many different problems. High nitrates are one thing to
check. Internal bacterial infections, including fish TB, are other
possibilities. If there are no water quality problems, you may want to
attempt antibiotic treatment in a separate tank.