I have about 3 4" and smaller plus 5 larger one in my 150 and they are ok. I feel that the smaller one are more active (more agressive) towards each other than the larger ones... but I gotta agree with Lupin.. two or three is about right....for that size tank...
So the next question would be how many i could fit in the tank, of a small size geo such as the [FONT=arial,sans-serif][SIZE=-1]Tapajo Red Head[/SIZE][/FONT]. i think they are one of the smaller types.
yes your right... I know some one who has about 4 or so.. 1 male and three females.. you might get a mini brawl for dominiance... but that should be cool...
I have 6 Heckelli about 6" long in a 120 (4'x2'x2') along with 4 small gynmo's and 5 angle fish. No issues. If they decide to breed I will be screwed but no issues. A pair of the small gynmo's did breed a few weeks ago and everything was fine.
I would do 6 or so red heads and you should be fine.