You're almost certainly misusing the nitrate test kit if you're getting consistent 0s. They are you notoriously finnicky. Are you using the API test kit? If so, you need to shake bottle #2 *before* you add its reagent to the test tube, then you also need to shake the test tube as you were probably already doing. When i say shake i mean shake the living hell out of it. If you're not using the API test kit i suggest you re-read the instructions and look for a small blurb you might have missed.
I'm betting that your nitrate levels are actually immense and you just aren't detecting it.
As an aside, 6ml excel daily seems like an awful lot for a 30 gallon tank, especially one with animals in it. Isn't the reccomended amount about half of that?