The concern is with one of the GloFish in particular, the Electric Green Tetra. It's a modified black tetra fish capable of living in freshwater. But as Adrianne Appel of the Washington Post reports, this one is not like the others:

The two GloFish are very different, however, in what environmentalists and some experts say is a crucial way: The heat-loving zebra fish is from southern Asia and can't survive long in cooler U.S. waters; thus, the Food and Drug Administration has ruled that there would be little threat of invasion of U.S. waterways if it were released from home aquariums. But the black tetra is native to South America and likely to be happy making a splash in the inland waterways of South Florida and Latin America.