how many peppermints per breeding tank?

Hey guys don't seem to be to much activity here these days but maybe doug can answer this one, currently got trio's of peppermints been slowly working out which ones are breeding and which arnt. They are in 3 x 18 x 18 tanks. Just wondering whether these tasks are too small to house 2 males and 4 females? or should i stick with just the trio in each? I do have a 5 footer with my excess pepps but there are at least two breeding males but about 6 males and 7 females in that one And looking to free it up.