Mangroves are trees, that grow very slowly. They feed off of the nutrients (poop, uneaten food, dying snails) in your tank. They work really well, but regular maintainance is required. They also eat up your Mg once established. They take awhile to begin working though. The red variety is the best suited for the aquarium trade. There is also black and white, but they're not as well suited for our aquariums. This is the cliff notes from the wet web media link in the first page.
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