If you want to use that bookshelf tank, some suggestions off the top of my head that will suit your grandson's small tank are a
betta, or a honey gourami, or a sparkling gourami, or an African Dwarf Frog, or white cloud minnow, and you can add to any of those a couple shrimp and/or a mystery snail.
You may want to read about the
nitrogen cycle if you haven't heard of that:
http://www.fishlore.com/NitrogenCycle.htm
I'm sorry your grandson is having to return his fish. That store clerk should have been more honest or better educated in order to help. That's great that he can have his goldfish in a good sized tank!
