Another suggestion (from my personal experience)-- take some sample water and your nitrate test drops to the lfs....do a side-by-side test -- meaning, have them test your water using THEIR test kit, and do your test with your test kit at the same time. See if you get the same results. wouldn't hurt :)
I found out that my nitrate test kit (which kept telling me I still had 20-40ppm of nitrates) was bad. I was battling with 160+ppm nitrates when I first set up (turned out to be ntirates in the tap--so i got a ro/di). Even after I made multiple changes, I couldn't get below 20-40ppm of nitrates. And I had no livestock except a few crabs/snails! Finally, I did a side-by-side test with my test kit and the lfs'.....turns out my test kit was bad! The lfs actually tested my water as having 0 ppm.
It was brand new (date on the box wasn't expired), and I had just opened it a month prior. So now, before I buy a new test kit, I test the new kit with the store's hehehe. Yours might not be bad, but if your water is good, then it might be the test kit. It wouldn't hurt. Just make sure to bring enough of your tank water.