The timeline:

3-17-13 setup new 12 gallon tank with water treated with Prime...temp 72-74.

3-19-13tested water with API master test kit pH: 7.4, ammonia 0-.25, nitrItes: 0, nitrAtes: 0... not sure what I expected to find, but tested anyway for a baseline. Added about 1/3 sm bottle SafeStart, waited 1 hr, added fish (3 small).

Continued to dose with SafeStart each day.

3-21-13 tested again...pH: 7.4, ammonia 0-.25, nitrItes: 0, nitrAtes:10-20!

I know the kit is good, have used it on other tanks.
How is this possible?

Will continue to monitor closely.