A 120g may not be big enough for 2 that are not a pr. In some cases the dominant one will drive the submissive one into hiding or the other side of a tank, but other instances they will kill the submissive one. This can occur in even huge tanks. In a 10ft 1000g aquarium a dozen firefish went in, and over the course of 3 months whittled themselves down to a pr that have now been there for 2 yrs.