The problem with a common name like harlequin rasbora is several different species may be sold under a given common name. I've seen at least five different species of synodontis sold as "upside-down catfish". Even the Shedd Aquarium in downtown Chicago lists different species under the same common name. At first, I resisted learning the scientific names because I've met people that turn up their noses at people who don't know the scientific names. However, I found that knowing the scientific names can clear up a lot of confusion.
If you're talking about rasbora heteromorpha, they're fairly big-bodied fish. I've got five in my 40B and they're 1.5"-2" long, pretty tall and thick, and really active. I started with them in my 20H and I had to move them because they looked cramped.
If it's rasbora hengeli, they'll be fine in a 10g. I've got six of these in my 20H and I wish I could find some more. The bright orange coloring is really attractive.
