Nitrite spike?

I'm not too sure, but there appears to be a nitrite spike in my tank.  This is tentatively good news. The way I understand it, the nitrites will spike, followed by a spike in nitrates. This activity should also result in a reduction of ammonia, which is mandatory to create a hospitable home for the fish.

For the record, I think that the spike in nitrates on the 29th in the graph was caused by the addition of some water from my other aquarium.