the pond
Is overall pretty stable on the nitrates and nitrites, despite the 100 fish in there. The enormous cattail plant gets all it needs and more. I toss in floating plants in the summer and I have to use a UV filter to cut down the algae load, otherwise it becomes pea soup.
Where I have to concentrate on the nitrogen cycle is with an experimental aquaponics system I've had running for almost a year now. Water from a fish tank is cycled through a growbed with a gravel medium. In the growbed are tomato plants, some onions, ginger, etc. It works pretty well, but requres more water changes and monitoring of the water. Still, it works pretty well.
EDIT: more info here http://en.wikipedia.org/wiki/Aquaponics