That is such a small tank that I would not add another species of fish. More Neons or Otos could work. Or a single top of the tank fish like a Honey Gourami.
But a 20 long just does not have the different zones that would allow another school of fish to look good. Nowhere for them to be that is not already inhabited by Neons.
But a 20 long just does not have the different zones that would allow another school of fish to look good. Nowhere for them to be that is not already inhabited by Neons.