As Nick pointed out above, quite astutely, just because the water shows zero doesn't mean it isn't being produced. It is just being consumed by something. My plant vendor is telling me the trick is to get the plants going well enough to consume so many of the nutrients that the algae don't have a chance. If you were producing it faster than the plants and algae combined could consume, then it would show up as an excess.
That's one of the awful things - the algae covers the plants, slows them down, and the algae gets more. Our test kits aren't that sensitive anyway. I wish I knew more about the cell metabolism of the algae, and how to tell exactly what this is.