You are using the nitrate test from API, correct? That test is notoriously tricky. Shake the crap out of bottle #2, bang it on the table, etc. for the full time x2. Then shake the test tube for the full time, and don't skimp on how hard you shake it. It is a tricky test. You could be getting a false low for nitrates. You may be more cycled than you think, especially with the addition of the mature media.
Test again when you get a chance and really shake it.
Yes, it's the API Nitrate test from their master kit. I shake bottle #2 for whatever amount of time it tells you to & then I immediately add it to the test tube. Then I shake that for a full minute & wait for at least 5 minutes for the color to develop. I'll try shaking it more vigorously next time & for twice the length of time.
I think the reason for the low nitrates might be the fact that it's a fairly large tank with only 5 tiny 3/4 inch long tiger barbs, 3 medium 1.5 inch corys and 3 gouramis that are around 2-3 inches long each. Also, I did do back to back 50% water changes just a couple of days ago. I'll test again when I get home tonight.