Even with Tetra Safe Start your tank will still take several weeks to cycle completely. The product says you can add fish immediately which is true, but it doesn't mean the tank will cycle that fast. If you go to the "Betta Fish Bowls, Habitats, and Accessories" section of this forum you'll see several sticky threads that explain the nitrogen cycle, I'll link them here.
Step by Step Nitrogen Cycle
Nitrogen Cycle - Betta Specific
Beginner's Guide to Freshwater Aquarium Cycle
Step by Step Nitrogen Cycle
Nitrogen Cycle - Betta Specific
Beginner's Guide to Freshwater Aquarium Cycle