Nothing until you get your tank cycled, and IMO that is not the best stocking for a 20 gallon,
Danios require a 3 foot tank minimum
Mollies require a 25 gallon minimum
The betta is fine
And PS, can you please be a little more detailed with your posts, because how Is anybody supposed to help you if you just say a Betta, mollies and a zebra school, you can atleast say how many yo have and the size of you tank
But once you have your tank settled and your sticking you can have fish like cardinals, harlequins, rummynoses, boraras brigittae, and many more small tetras, and small centerpiece fish but it depends on your personal prefence