They're wild rebels and they go behind the rocks n plants and do naughty things back there. No wonder I took'em out of the community tank and gavem their own, they're bad news!
Interestingly enough, there is no "wild-type" for goldfish as they were created from the Crucian Carp (Carassius carassius) and artificially evolved to their current specific status (C. auratus). Fun fact for tonight!
How many goldfish do you have in your 10 gallon tank. If you have more than 1 it is overcrowded. 1 per 10 - 20 gallons of water would be good. I would think that the are stressed. I know I made the same mistake when I first started with my goldfish tank.