[Diana]

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.

 

[Aurie]

personally I'd get 1 more otto, they seem to like groups.

I'd suggest something small, maybe a couple nerites or a mystery snail

 

[Nestle_]

I completely disagree with any gourami being added here.

more oto's would not be a terrible idea, but I think you are near your limit on fish.
Especially as the GBR become active breeders.... things will get a bit tense in the tank

 

[2pac]

yeah no gourami. like others have said more otos otherwise you're pretty good

 

[Patriot17]

Thanks for all of the advice everyone. I think if I do get anymore fish it would just be another Oto like most of you suggested.