well R. mangle should be the right ones that can take the full strength saltwater.
As for being worried about chemicals in the gravel etc... I would probably take them out of the gravel and rinse them really well in fresh water.
They are pretty hardy for changing salinities... some people just plop them into full strength saltwater and they do ok. I would keep it in a separate little bowl and rinse it with fresh water and and swap out its water for a couple of days to make sure that you have removed any fertilizer or other chemicals from the plant, and then slowly add saltwater from your tank to the little pot to make up evaporation until eventually the salinity in the little pot with the mangroves matches your tank, and then just move them over.
If you are already doing partial water changes, use your change water for making up the evap on your mangrove, that way it gets a little fertilizer in the form of wastes in your water from your tank and you are recycling that water a little
.
Once the mangrove is in saltwater, it will require magnesium in the water to allow it to exclude salt takeup from the roots... so if you already have things eating up your magnesium levels in your tank, you may need to dose a little every now and then to make up the magnesium.
Also, mangroves in saltwater need to have their leaves sprayed down with fresh water every now and then to remove salt that they excrete from pores on the leaves.
Also, I read that mangroves don't like their roots being disturbed, mine I got as little starters, so they didn't have a lot of root structure to hurt. Not sure how big the ones you got in the gravel are, so be careful removing them... you don't want to damage roots any more than you have to.
Good luck,
Doug
