cichla kelberi or intermedia.
alot of debate about the eventual size of kelberi.but they get below 20".
intermedia get below 20" as well.
both species are not very easy to find and can be very expensive although it seems more and more kelberi are readily available at low prices.
tank size IMO for 2-4 individuals is a 6x2x2.
bigger with more bass.
as with all cichla,keep the temp. high like 87-90.
regular water changes.