Robbed directly from the Glofish website:
Exactly how is the fluorescent protein gene added to the fish?
Every line of GloFish® fluorescent fish (i.e. Starfire Red®, Electric Green®, Sunburst Orange®, and Cosmic Blue™) starts with a single fish. The general process of developing fluorescent fish, as illustrated in this chart
, begins by adding a fluorescence gene to the fish before it hatches from its egg. Once the gene integrates into the genome (i.e., genetic code) of the embryo, the developing fish will be able to pass the fluorescence gene along to its offspring upon maturity. Because of this, the gene only needs to be added to one embryo; from that point forward, all subsequent fluorescent fish are the result of traditional breeding.
This would suggest to me that they are in fact not infertile. However they are bred under license and the deliberate breeding and selling of Glofish by anyone other that Yorktown Technologies (or their partners and subsiduaries) is illegal. The fact that they have had to add this codicil would also suggest to me that these fish can breed.