I here monos are hard to keep.. is it?
No, they are not particularly hard. All they really want is a big tank, very hard and alkaline water, plenty of water movement, and at least some salt in the water. They'll eat pretty much anything meaty as well as flakes/pellets, and in terms of social behaviour they are pretty standard schooling fish: sometimes aggressive to one another in twos or threes, but in big groups (six or more) completely peaceful. There's some anecdotal evidence that adults "pair off" like discus or angelfish and become territorial, but this doesn't always happen.
My own experience is three M. argenteus, with one a piece of M. sebae, common archerfish, and the silver scat Selenotoca multifasciata, and three Colombian sharks. These lived in a brackish water tank of 200 gallons with a big wet/dry trickle filter and a protein skimmer, at around SG 1.010, and periodically shared their home with a spotted gar and various tilapias.
They do grow fast in the first year or so, and then slow right down. I'm told they can get pretty big in the wild (20-25 cm) but this doesn't seem to happen in home aquaria, even fairly large ones.
Cheers,
Neale