Generally, that indicates some kind of stress on the fish. Causes of stress can be all kinds of things like poor water quality, overcrowding, etc.
It doesn't hurt to throw a few boxes of kosher salt into the pond as a mild boost to fish health.
Excerpt from Robert Crais' "The Monkey's Raincoat:" "She took another microscopic bite of her sandwich, then pushed it away. Maybe she absorbed nutrients from her surroundings."
