can a Rhom live in a 55 gal with undergravel filter and one hang-on-back-pour-in-type filter for life?? and if so how long will it live? how big will it get max?? this would be the only resident in the tank
a rhom could live in that size tank for quite a while probably upto 8'' maybe a little more.
i would not get a undergravel filter through p's are messy eaters and food can get trapped under the base and deteriorate without you knowing.
as for max size on rhoms they do get big but take a really long time some people on the boards have 17'' rhoms but usually get them that size due to the time period to attain large sizes (years).
give it some bogwood and gravel and plants to feel secure and it will be ok.
i would say an elongatus in a 55 would also be ok for a while but a 75 would be better as its wider .
they do reach @10-12 '' max and are a real live wire fish (they are built for speed and ambush) so a 48lx18d should be ok for life.