Fishkeeping - new goldfish going down the tubes -- help?

Had two goldfish for four years without problems. One died recently, bought
a small replacement (about 1") from Petco on Monday. Placed newcomer in
separate tank.

Monday - Thursday new fish was OK, although not terribly perky, but
basically normal.

Thursday PM: fish suddenly sickened. Symptoms haven't really changed over
past 3 days: Fish very weak, makes occasional swimming motions but often
just floats up and down in tank. Unable to swim upright, turns around and
around. Sometimes lies on bottom, sometimes in middle of tank, sometimes up
on top.

Thought he would die within a day, but this is third day like this.

Water quality is fine, and has been changed a couple of times.

This isolation tank only has a bubbling air stone, nothing else.

Any thoughts? Thanks

Michael