Jim,
From what I have heard, they are very territorial. I think we tend to get away without any problems at the shop, as we keep them in such a quantity (50+ specimens in the stock tanks).
Did you see this recent thread on petfrd?
http://www.petfrd.com/forum/showthread. ... ge=1&pp=15 Mention is made of their aggressiveness there.
Having acquired four of the similar species
M. guentheri not so long ago, I can (sadly) confirm that these chaps were highly aggressive, and despite being in a fairly large tank, they fought so much and gradually picked each other off one by one overnight. I think the trick is to either keep one, or lots.
Here's some photos of the
M. triangularis at the shop:
Despite their cuteness factor, I would advise caution with these!
Emma