I would say that you could keep one male histrix in a 120 for life but, I would not suggest it.
Histrix to stay comparatively small and their tails are also short (one of the reasons that they are still my favorite ray).
Dave...the big male was purchased as a 3" pup in the Spring of 2001. The smaller of the two females came from Aquascape as a pup in the Spring of 2002 (she killed the male that came with her). I do not have much history on the largest female.
They look great. You have done an amazing job with them. I am thrilled that more and more people are getting to keep histrix. Dave's breeding of them is a huge push in the right direction.
All the new histrix postings are just awesome. For a while I was really worried that they would virtaully become "extinct" in the States.
Colin