Tim what exactly do the mangroves do? Do you have them in your sump or where?
Micki, I attached a few links for you. The first link is pictures of mangroves from the wild (IE, they protect the reefs by exporting phosphates and nitrates as a food source and they also protect the coastline from erosion, etc).
Mangroves are slow-growing plants, and their nutrient uptake is limited in the home aquarium do to size limitations (IE, most people cannot grow large trees indoors w/ massive root structures, etc). In the wild, the mangrove root structure will absorb lots organic material, phosphates, nitrates etc. from the water.
http://travel.mongabay.com/mangroves_photos.htmThe second link is an article about mangroves in the aquarium. I keep my mangroves in my 125 gallon macro sump.
http://www.advancedaquarist.com/issues/apr2002/feature.htm