I used to keep monos in my planted freshwater tank.
Basically, when they are young, they are bred in freshwater to replicate a natural situation. Mature monos (both argentis and saebe) will go into the freshwater and brackish deltas to spawn. As they grow old though, they prefer brackish to saltwater since they move out of the shallows when aging.
In my opinion, monos are ok to be in brackish water at a young age as long as they are properly acclimated, but attempting to keep mature fish in freshwater their whole life stresses them.
Another thing to consider is that they usually like lots of coverage, easily provided by large plants in freshwater tanks. Since they do grow large, I am not sure that rockwork in a SW
aquarium would suffice their need for cover when they grow up which can also stress them. They do have quite a bit of character though, and mine was nearly trainined to be hand fed.
I am not so sure that I would attempt to put a juvinile in pure saltwater though, never experimented. Just providing what I do know about them.