read on OPEFE that for rhombeus a square tank would be better than a long tank, 120g would probably be fine if you don't have the space for a 180g. http://opefe.com/rhombeus.html
"IN THE AQUARIUM
Recommend keeping as solitary species in home aquarium. Large specimens like the one above should be kept in dimly lit aquariums. These larger one's inhabit deep waters where sunlight is less likely to penetrate. They are river fish and require strong current. I further suggest keeping them in a deep square tank than a long tank. Dimensions should allow sufficient turn around for the fish so that it will avoid bent tail syndrome, a feature seen on some fishes where the bottom or top of the caudal fin is crooked. This sometimes occurs from a fish being kept in a container that is to small for the fishes requirements. A power head is a must for flow and to keep the fish healthy! Water temperature should range from 76-82F. Soft, slightly acid preferably between pH 6.8 to 7.4. In a topic further below, I have included additional information on water chemistry reader's should consider."
I'll try the powerhead idea i just pickup a true blue diamond about 6 in. I've got it in a 150 gallon. Plus lots of water changes is one way yo help increase size fast.