Dick wrote in
:

In my experience, odd swimming behavior was probably related to a
failure of the float bladder. I have never seen it spread to other

Swim bladder damage is another possibility, however the fact that the fish
can only swim in a circle suggests that there is muscle or nerve damage
involved as well.

Difficulty staying neutrally buoyant is usually more typically akin to swim
bladder problems, but that can also be due to gastro-intestinal problems as
well--a condition common to goldfish. Usually if the condition is
transient the later is responsible. Increasing the amount of vegetable
fiber in a fish's diet can often help with gastro-intestinal conditions.