Before I continue I must comment... Your avatar is awesome!
The nitrite is coming from either the ammonia or the nitrate. Typically, it flows from ammonia to nitrite to nitrate, but in anaerobic conditions it may flow from nitrate back to nitrite. Any spots that accumulate a lot of organic matter with little water movement could be breeding grounds for some anaerobic bacteria.Originally Posted by jrlevers
Otherwise, the legumes (beans and peas and such) are not your source of nitrite. Nitrogen fixation from these plants/bacteria yield ammonia.
N2 + 8 H+ + 8 e? ? 2 NH3 + H2