Well, I will be interested to see what other people say, because I have almost the exact same thing going on. Its a 10gal tank, and I did not add any ferts, but otherwise the story is similar to yours. Seeded filter, and been cycling 2 days.
In my case, I know there are nitrates in the water. However, I have had that water tested and I know that they are at 10-11 ppm. And I have a new fancy (read: expensive!!) RO system for our drinking water that the dealer claimed would remove 90+% of the nitrates. I used the API test kit and got the following results:
aquarium water (40 ppm)
well water (40 ppm)
RO water (40 ppm)
distilled water (0 ppm)
At least API got the lack of nitrates correct. But I am suspicious of the other readings. The RO water, which should have lower nitrates, took the longest amount of time to turn red/orange. But it still eventually turned the same color as the other samples (about 15 minutes, as opposed to 5 for the other samples).