Many fish have an iridescence that causes them to reflect light. Some tetras look as if they have electric lights inside them such is the dazzling effect. As the fish moves through the water their colours can change as the sunlight (or tank light) catches them at differing angles.
This iridescent effect is caused by crystals of guanine that form just beneath the scales. Like tiny prisms they split the light and reflect it back in rainbow colours. Some fish have evolved bright stripes and patches of colour which they use in display and as signals to others of the same species to help keep their shoal together.
Oddly, it may even help some species to conceal themselves from predators – as the light is reflected away from them they can blend into the background better.
Fish keepers have long noticed that fish in good condition will have more iridescence whilst poorly fish will look dull. Production of guanine is clearly not without cost and when fish are ill they will divert all their resources to fight whatever is causing their illness. A well fed and healthy fish can afford to divert energy into guanine production and as a consequence will positively glow. All the better to impress the ladies no doubt!
References:
- Fish scales hold dazzling secret –Royal Society of Chemistry
- Fish Iridescence for Protection – Fish Channel.com
- Guanine definition – Answers.com
UK Aquarist 