I myself am messing with Mangroves right now. I have had them for about a week and a half. Too early to tell now though.
What I have found on them is this:
They are a freshwater plant/tree.
They are able to filter the freshwater out of the saltwater to live.
They perform very similar to an RO system in filtering the salt out.
They use magnesium to pump the water through (something like this).
They need iron, magnesium to thrive. They consume the nitrates, etc to live.
There is another site for mangroves
http://www.athiel.com
This person has some pretty interesting statistics on them and the results of some testing he did with them. Impressive results with the testing.