There are a few natural species of glowing freshwater fish. One example is the Japanese freshwater green eel, which contains a fluorescent protein in its body that causes it to glow green.
Glowing freshwater fish that are purchased in a pet store are genetically engineered to glow, and many species of fish are available with added bioluminescence. Bioluminescence is natural light produced by chemical reactions occurring within the organism, and it is the primary light source in dark waters. Bioluminescence is of little benefit to fish found in freshwater because of the often murky water conditions. Freshwater habitats are not as ancient as saltwater habitats, and luminescence has not developed as a necessary part of biology. However, there are many creatures that utilize bioluminescence in the ocean including bacteria, single-cell algae, squids and fish. In nature, bioluminescence protects animals from predators and helps them locate prey and attract mates. It also lights the way in the dark depths of the ocean.
Another freshwater creature that glows in the dark is the latia neritoides, a small limpet-like snail found only in streams of New Zealand. Scientists have been studying this animal since 1968, but its unique system for producing bioluminescence remains unclear.