Thanks for responding. I did look quickly at the fish, and I noticed
a purplish discoloration around her abdomen. I couldn't tell if she
had been injured or was sick, or whether maybe she had been dead a
while and the color change meant she was beginning to decompose. Any
thoughts about that?

Thanks for the link to the Fish Physical website. That's great to
have!

Joan

next time do a quick physical, especially looking at slime coat and gills.
http://puregold.aquaria.net/pg/disease/technique/technique.html#Jo_Ann's_Fish_Physical
when nothing shows on outside and no symptoms it is "sudden death" and this is fast
changing water parameters, like loss of oxygen or toxic spike ... or, it could be
internal bacteria. Ingrid