Monos can be found in fresh and brackish when young, as they mature they tend to stay in brackish and full sea water.
I agree with skyddakyd, other than temp, the African rift lakes are hard and alkaline not salty (as in NaCl) very much like our great lakes.
The cenotes along the coast of Mexico are salty, I wouldn't hesitate to add salt to a tank with JDs, sailfin mollies, or sleeper gobys from there.