Dan, Sounds like another success. I use the Lamotte kit for nitrates which is clear when at 0. The API kit has a color for 0 so I wouldn't be surprised if you are at 0 and just getting a little fluctation in the color reading, anyway less than 5 is a success. You might want to check your phosphates also.
I add different bac sources also. I am currently adding MB7, but I have added ZEObak previously.
It makes no difference how much I feed, I haven't been able to read any nitrate since the day after starting the pellets 2 months ago. As I have said many times my phospahtes do fluctuate though, although not that far any more.