Mangroves are not nearly as fast growing as macro algae if you want them to suck up nutrients in the tank. You would need to use a ton to get any appreciable effect - also they are sensitive to fluctuations in salinity and do much better in brackish - fresh water ime. If you do get them you'll need to mist them periodically a few times a week or daily as salt collects on the leaves and exposed stems.
Also you can 'plant' them in a substrate and they do lateral roots or place just the root ends in the tank and slowly raise them as they grow to get the dramatic prop roots to develop. There is a good chapter on them in Calfo's first book and Julian Sprung wrote a neat article on their use in aquariums you can find on the web.
Info I found from AA by Daniel Knop on mangroves:
http://www.advancedaquarist.com/issu...02/feature.htm
Here is the .pdf of Sprung's I was referrring to :
http://www.twolittlefishies.com/docu...1100232022.pdf
FYI I had about a dozen mangroves in my sump and they did ok for a couple of years till I moved them and they started to drop leaves and shrivel. I am told they have a significant magnesium uptake of which I seem to be chronically low on.
30 Gallon reef, 220 gallon South American Cichlid tank.