I tried to put two Gramma's in a 180 with a lot of rock, research I had done seemed to indicate that even if they didn't pair up they would have enough room to inhabit the same tank. Within moments they did this lethal spinning lip lock buried down in the rock where I couldn't net them. Both died. I hate these learning experiences... Never kept the pseudo's, easier to establish a mated pair from what I have read, but a single one is supposed to be very agressive towards even larger tank mates, I can only imagine how a mated pair would behave.... I have kept single Grammas with great success in "community" tanks over the years, I think you would have no problems with a single gramma for the most part, getting a pair would probably be a challenge, if you succeed I'd love to know how a mated pair behaves, might be a whole new aggression thing possibly.......