I have to mention something... they look so bright and lovely in the pet store, but once you bring them home, they won't be. They're genetically "engineered" to "glow" when water conditions are bad. That's why they're so bright and colorful in the stores. I bought one, and it wasn't glowing at all (even under black light) after a few days.
They're also aggressive. It was ridiculous. This one bullied all the other fish, fin nipped, and was just a butt head. The others finally hid for fear of its wrath.
They stay at the top of the tank. Never saw mine go remotely close to the lower middle/bottom of the tank. They really enjoy the verrryyy top. They're QUICK. If you want your other fish to be fed, don't get this species. They're fast, they're pigs, and they will bully others away from the food.
Oh yes, mine was nice too. He was for weeks (maybe 1-2 months). But once it got settled... the aggressive/bullying behavior came out. Severely.
It also stayed sick. Its stomach stuck out so far I thought it had 100 worms in there. All my other fish? Perfectly healthy. It was just that one fish.
It's illegal to sell them, give them to pet stores, distribute them, etc. After I got it, and I didn't want it anymore (I only got one since I had a school of other fish its size already, please don't just buy 1-4 if they're by themselves, they NEED 6+) I had no way to get rid of it. My mom got so sick of it that yesterday while I was taking the other fish (who were starved, fins ripped apart, and scared) to the pet store that would take them flushed that Glo. I would've culled it, but, too late!
If you get GloFish, they'll need to be the only fish. 15 gallons+ with a minimum of 6. They get to be 2+ inches!
Those little babies you see in the store? Don't stay that for long! I'd suggest a 20 gallon with 8-10 Glos. A 20 gallon long will make it look more "filled", if you got a 20 high, the whole bottom half will look totally empty.
Just keep these things in mind... they're not the perfect little guys. Mine turned into a total disaster and it ended badly. I even researched before hand. It just didn't turn out the way I expected, and I couldn't deal with it.