I should caution you on my own experience. I received some mollies that were several generations bred in soft water. They were fine as they were, but I wanted to maximize their breeding potential, so I slowly took them to light non-marine salt, hard-water pseudo-brackish. SG was only about 1.002/1.003. They did well and grew a lot in this set-up. Later, I played musical tanks and took them back to soft fresh over the course of a week doing small water changes with normal dechlorinated tapwater. Once I moved them into my 55g tank, though, they began to show signs of stress. Fins clamping and small white areas appearing in their slime coat. I gave them a dip in full-strength marine mix and they recovered completely, but I have no idea if/when I'll have to repeat the process. So go easy on the transition from brackish to fresh. Seems to be a tougher change than fresh to salt. At least, for those few young fish in my own care.
