Like all bacterial additives, you need an ammonia source. If you add fish to provide this source you are stressing your fish. Add pure ammonia or a piece of raw shrimp and then add Fritz 9. You will need to measure your ammonia and nitrite. Once they go down and you see a nitrate spike you can do a small water change and then add fish. This cycle takes at least a week. Fritz 9 only gives the nitrogen cycle a jump start.
Directions State - Allows Safe Immediate Introduction of Livestock. I Added Fritz Zyme to a new aquarium fallowing directions. After 2 hours ammonia and nitrite were still to high to add fish. This possibly due to Fedex delivering a day late and allowing the bottle to freeze.
It clears the bacteria but you have to use it every 5 days or so