You could probably put one more danio, doesn't have to be a glo fish. I have one glofish danio and she schools with my 13 other assorted danios. (Don't tell the cops... Haha) apparently they're illegal I'm my state. I didn't know I was buying a glo fish until I got home and researched.
So, this is what I would do, smaller tanks are harder to maintain because there is less water to dilute the amonia, nitrites and nitrates so I would plan to upgrade in the near future to a ten gallon. They run like $30 for a starter kit and your fish will be happier and the tank will be a little easier to maintain. In a ten gallon you could have the three danios, some ghost shrimp, and a couple of nerite snails. Even better! A 20 gal, petsmart and petco have the $1 gal sale twice a year. I know it's a lot to take in but a 5.5 is really not a happy place for danios. They like to swim and swim fast!! They'll live and probably do fine, but if you have the room, please consider an upgrade. You could still use the 5.5 cube for a qt spot.
The pet stores were not much help to me either. Most of what I learned I learned from these lovely people.
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Feel free to ask questions. 
