Mangroves

Mangroves are cool, but you'll have to set up a rather specific in-line system to keep them. Also, they're slow-growing, so their effectiveness as a nitrate reducer is a bit overblown. For denitrification, you'd be much better off setting up a refugium with various macroalgaes IMO.

I agree. Mangroves are cool, but they grow out of the tank, so need to have a bright light source that does impede vertical growth. Not the best thing to have in your show tank! Many people set them up in a refugium, were they can provide the light via a pendulum style light, instead of a enclosed canopy like most people have on a show tank. Drs Foster & Smith usually have them for sale on their webpage, but you'd be better off using macro algae or even xenia instead.

However, it's important to note that high nitrates usually indicates other high toxin level and low mineral levels in an aquairum. A water change schedule that keeps nitrates low will also replenish these other additives (like calcium, buffers, ect), creating a more stable environment.