I guess there weren’t enough colors in the ocean.

When scientist first started working to genetically modify Zebra fish, it was in the hopes that a small mutation would allow the fish to identify certain pollutants in waterways wherever they were introduced.

In 1999, Dr. Zhiyuan Gong and his colleagues at the National University of Singapore extracted the green fluorescent protein (GFP) gene from a jellyfish that naturally produced bright green bioluminescence. They inserted the gene into the zebrafish genome, causing the fish to glow brightly under both natural white light and ultraviolet light. Their goal was to develop a fish that could detect pollution by selectively fluorescing in the presence of environmental toxins. The development of the always fluorescing fish was the first step in this process. Shortly thereafter, his team developed a line of red fluorescent zebra fish by adding a gene from a sea coral, and yellow fluorescent zebra fish, by adding a variant of the jellyfish gene. Later, a team of Taiwanese researchers at the National University of Taiwan, headed by Professor Huai-Jen Tsai (蔡懷禎), succeeded in creating a medaka (rice fish) with a fluorescent green color.

The fish were first introduced into the U.S. market in 2003 after FDA approval:

Because tropical aquarium fish are not used for food purposes, they pose no threat to the food supply. There is no evidence that these genetically engineered zebra danio fish pose any more threat to the environment than their unmodified counterparts which have long been widely sold in the United States. In the absence of a clear risk to the public health, the FDA finds no reason to regulate these particular fish.
- FDA

And like most research, it was commercially exploited before it was fully developed. Which always helps the continuation of funding for the research, and, in this case, give us a tank full of fluorescent colored fish to enjoy.

Unmodified Zebra Fish

Zebra Fish as Pollution Indicators

So far, the researchers have succeeded in isolating two types of gene promoters in the zebra fish — an estrogen-inducible promoter and a stress-responsive promoter. These promoters have been used to drive the fluorescent colour genes in transgenic zebrafish. Such fluorescent-coloured transgenic fish will be able to respond to the presence of chemicals like oestrogen through the estrogenic promoter and heavy metals and toxins through the stress-responsive promoter. The fish will immediately display the colour depending on the type of environment the colour has been specified for.

Although only red and green colours have been produced in the zebra fish, A/P Gong revealed that he could add up to as many as five colours to the zebra fish, each colour to indicate a different pollutant. In using such transgenic fish, pollutants can be detected with one quick look. The fish are also biodegradable and economical to breed. All these factors make them very suitable pollutant indicators.
- Zebra Fish as Pollution Indicators

Images from GloFish and Wiki: GloFish