That is strange. I was always under the impression that mangrove are brackish water plants - in fact many grow them in saltwater sumps or refugiums. Perhaps different species have different requirements? One thing I do know about them is that they have a huge element uptake, calcium I believe, but don't quote me on that. I know that several who tried them in salt tanks around here reported having to hugely up their dosing in order to maintain desired levels once the mangroves got any size to them.
I think they definitely do best in hard water with lots of minerals. It might have been that I just did not acclimate them slowly enough. Some kinds of brackish-adapted plants, such as white mangrove, occur further away from the ocean water and thus only tolerate slightly brackish water. Some others, such as red mangrove, can grow in very salty water. Reef hobbyists grow red mangrove in full-strength seawater.
In general I think it is just easier on the plants to have somewhat less salt, so for these brackish riparium plants I would recommend less saline brackish water.