The original Glofish were zebra danios genetically modified (GM) with genes from jellyfish. They were not developed for fish keeping, but rather the scientists were trying to develop a fish that would selectively fluoresce in the presence of pollutants. This was an overall failure, but the glofish industry was born.
There are now multiple varieties besides danios, including GM white skirt tetras, and more recently tiger barbs.
These are not dyed or tattooed fish, and they will breed true (meaning keep the color in subsequent generations). There have also been reports of them breeding with regular non modified danios, and the offspring have some of the color but not as intense. They seem to have decreased breeding ability overall though.
Glofish are only sold in the US, with the exception of California. California has a ban on GM fish of any kind.
I purchased six of them 3 years ago, because my kids wanted them really badly. Of the original 6, one remains, and appears quite healthy. I will not purchase any more, now that I know what I know. I have an ethical issue with GM strictly for aesthetics.
As Paul Harvey used to say, "and now you know.... the rest of the story."
