Agreed, betta or ADFs. Having 5 harlequins in a BiOrb just.. isn't a good idea.
As pretty as a BiOrb is, it's possibly the most expensive, least useful fish tank I've ever seen. It's difficult to see through it because it's completely round, there's small surface area for gas exchange, and the filtration is crap. See if you can return it, say the filter doesn't work or something. Use the money to buy a larger, rectangular tank. Then you have more stocking options.