Bro, Red mangroves are unique and one of the hardiest plants that can survive in all types of salinity condition. When in seawater, they survive by excreting salt through their leaves. In home condition, we need to intervene by spraying the leaves with some fresh water every other day. I am using DI water to lightly spray onto the leaves twice a week. In nature, this action is done by mother nature as when it rains, the salt will automatically be washed off the leaves.
U will be surprise at the amount of nitrate it can pull out from the water....
Enjoy your mangroves and the natual biotope...