Fresh water? If so no I am pretty sure the lowest salinity would be a brackish environment. 1.007-1.010 I saw this post and was looking for the info I had on mangroves but cant find it. I lost a lot of saved info and sites when my original comp crashed a couple months ago.
mangroves can be used in freswater. but you will be better off using water hyacines. they take out nitrates faster than most plants. another choice would be water lettuce but it tends to grow quite fast and be a pain to remove.