It would appear that some decade or so ago fish were bred with special qualities that helped them detect pollutants. These qualities imparted to them a glow-in-the-dark quality which, well made them glow in the dark.
The fish were injected with a phosphorescent protein which allowed their skin to glow in the dark. The proteins are natural proteins derived from other creatures that have the same protien.
Or so Mr. Burns would have you believe!