Well, zebra danios are very active fish and even though they're fairly small, they would really appreciate more swimming space than a 5g tank can provide. They're also schooling fish and would do best in a group of at least six.
If I were you, I would return the danios and get some fish that would be more appropriate for a 5g tank. Here are some options:
1 betta fish (they are very aggressive towards one another so only one can be kept in your tank)
1 or 2 dwarf puffers
2 or so sparkling gouramis
1 male 3 female guppies
2 male 4 female Endler's livebearers
6 or so small tetras like neons or ember flame tetras
6 or so pygmy cories
Some red cherry shrimp
You might be able to do some combinations of those fish. For example, I have 6 pygmy cories and some Endler's livebearers in a 5.5g tank.