Well, I have no exp with mangroves but from what I've read they basically do a pretty good job of finishing up the nitrogen cycle, even to the point that the skimmer will stop producing skimmate due the water cleaning qualities of the mangroves. I have also, heard, however, that they can make water a bit yellowish, which stands to reason as these are plant, not algae, so they need to be considered as plants. [They are also freshwater plants, BTW, that need to be accilmated for marine aquarium use.]