It needs to be transitioned to brackish or even full marine at some point, which means adding marine salt. Salt alone doesn't replicate brackish water, it's best to get the marine salt mix to do it. That said, if you are set on that, just pick other fish that will transition to brackish as well. I don't have any recommendations for that apart from maybe mollies, sheepshead minnows, flagfish (could be wrong on that one), but whether they'll mesh with the monos is to be determined.