You are probably getting zero nitrates since aquatic plants prefer to take up ammonia rather than nitrate as their nitrogen source. In fact most aquartic plants will expend energy to convert the nitrate back to ammonia for their use. This is as opposed to terrestrial plants, a lot of which nitrates are their preferred nitrogen source. Anywho, your plants look really healthy, they are probably consuming the ammonia directly for sustenence, which means that there is no ammonia for the cycling bacteria. This is fine so long as you keep the plants healthy. But, no ammonia going thorugh the cycle, means no ammonia is being converted into nitrate, which is probably why you got the zero reading. So long as those plants remain healthy, your tank is going to be fine.
In fact, with healthy plants, you can run a tank filterless. There is a great book by Diana Walstad, Ecology of the Planted Aquarium that explains how to set up a filterless aquarium. The word filterless is kind of a misnomer, however, since the plants are really acting as the filter, in this case. Don't let people tell you that you can only have a very light stocking, either, because so long as you keep the plants healthy you can have as heavy a stocking as any other well-filtered tank. In her book, Diana has a 40 gallon tank with 55 guppies in it, and another with two large schools of rainbow fish.
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