Weeell...do you want mixed gender, all female, all male? How planted is the tank? How long has it been cycled?
And what exactly does excellent filtration mean...how many GPHs?
If you answer these questions, it will be easier to give you a correct answer.
From what you said...I'd say maybe 4-6 females and 2 males. This won't fill the tank up, but should you get anymore I think it will just be a little crowded. Tall tanks don't give them much length. Also, you have to account for some fry, which you will almost definitely have.
Or you could always go for 2-4 females, 1 male, and some Cories and maybe a few schooling fish. It depends, do you want other fish?