Tetra SafeStart is intended for a fish-in cycle by the manufacturer. Although I used it as heavy seeder for my fishless cycling, since I believe it is more humane.
If you are using API Freshwater Master kit, then it is possible that you have messed up the nitrate test. The 0ppm you got is a false reading.
The API kit's nitrate test is a little tricky. It requires extensively shaking before using, and then it requires more shaking after mixed it up before you can get an accurate reading. Fail to do so will result in 0ppm nitrate reading.
It is actually been a while since I used my API kits. For more detailed information you need to read the instructions on the bottle.