The last two posts here, #9 and #10, hold the answer to your question. Like all characins and cyprinids, this fish is a shoaling species and it must have a decent -sized group. Some species seem to manage with fewer than 10, but a scientifically-controlled study proved beyond any shadow of doubt that all shoaling species in these two families are going to be in better health if there are 10 or more in the group.
That study showed that increased aggression occurred when numbers were three and five, but not with ten. There were some other interesting observations involved with group size, and one of these was that the fish in the larger group (10) more readily fed well, whereas those in the too small groups did not. There is certainly no doubt that numbers matter to fish health when it comes to shoaling species.
On a different note, and picking up from Bruce...we as responsible aquarists should never purchase these modified fish. There is only one way to stop this practice, and that is if no one buys the fish. The UK and EU have prohibited the importation of modified fish including Glo-fish. North America is way behind in not taking action. Please be responsible, and do not encourage these monstrosities. At a time when natural ecosystems are being destroyed and species going extinct, we should be working to protect.
I am going to start mentioning something else more often, because this is a very serious issue for the hobby. Animal-rights activists are gaining strength in their attempt to prohibit the keeping of fish, along with amphibians, reptiles, birds, and mammals. This sort of flagrant nonsense (modified fish) only fuels their fire. I agree with and support any effort to end the global trade in rare animals, but this can come back to bite us.