Bio Spira is the only true instant cycle product. I have used it 3 times with excellent results. You add the bio spira at the same time you add a full bio load of fish. My experience was always exactly as expected. The last time I used it was in an emergency situation in a 100 gal. tank, and I had to move all my fish to a 55 temporarily until the bio spira arrived 3 days later. I drained the 100 gal. after removing the fish, added new water, and let the filters run. When the bio spira arrived, I added it to the tank, and immediately added my 2 oscars, 2 jack dempseys, and 1 large pleco back into the tank. That was in June 2003, and I haven't had a problem since. Stress Zyme is not an instant cycle product. It may help accelerate the cycling process, but I would not recommend using it with a full bio load of fish. If you have another tank that is already cycled, you could also borrow some filter media, or gravel from the cycled tank to help you cycle your new tank.