I would think the .05 nitrites would be due to the small bio load that has been placed in the tank recently. A few dead snails... new cleaners pooping, etc.. that should make a very small "mini" cycle that is going on now.
That is the reason you only add a fish or two at a time. If you were to add a lot of fish all at once your bio-load would be huge and the bacteria in the tank couldn't keep up. When you add a small CUC or 1 fish, there will be a little bio-load, and the bateria in the tank has the then compensate/grow to meet the new demands of the tank.
Thats my opinion anyway.. anyone else have their own ideas?