Had been keeping mangroves for quite some time. If view from the efficiency aspect, i do agree with AT that marine algae does absorbed nutrients at a faster rate than mangroves do.
However, mangroves are not really slow absorber as the amount of new leaves developed = amount of nutrient absorbed. A mangrove ecosystem provide a totally different biotope altogether. The process of how the roots forms and grow, how it absorb nutrients along the way, etc also provides an immersely intangible educational value. A mangrove filtration system can be an unique and interesting display by itself.
Attached is a link by Julian Sprung on the care of mangroves. Enjoy and have fun.
http://www.twolittlefishies.com/documents/1100232022.pdf