Similar shaped rainbow fish would be a Melanotaenia praecox (Dwarf Neon Rainbow)
I had the Dwarf Neon in my 29G. It was just barely enough room for them, for they are fast swimmers.
I think threadfin rainbow would work in the 20G. For minimum size tank for them is a 10G.
So you had those rainbow in a 55G last year. Did you downgrade to a 20G long? Why did you go from 55G to 20G?
Last edited by Hilde; 11-27-2012 at 04:18 PM.
Reason: added text