text: Color.  (Or if they're British fish, 'Colour'.)\n\nThere are some famous experiments on color constancy in goldfish done by a professor named David Ingle.\n\nHere's a picture of the cover of Science magazine showing Ingle's goldfish in front of some color swatches (scroll down).\nhttp://www.rowland.harvard.edu/organization/past_research/fishlab/fishlab.html