africangrey, you're correct there. There's no way a mangrove is going to compete against macroalgae in regards to nutrient uptake. In the time a mangrove grows a new leaf, you could have a few pounds of chaetomorpha on your hand. I think the main reason people have them is that they're a beautiful plant with interesting characteristics. Once they get bigger, their roots could shelter organisms for the refugium or tank, but as a filtration source, they'll never compete too well against macroalgae or a skimmer.