This bottle I have says:
"Contains live bacteria
speeds the development of the biological filter
helps keep aquariums clean
BENEFITS:Contains millions of live bacteria that speed the development of the biological filter to eliminate ammonia and nitrite.
WHEN TO USE: Whenever setting up, or to maintain fresh and saltwater aquariums.
DIRECTIONS:
Shake well
New aquariums: First, use STRESS COAT to remove choline and chloramines. Next, on the 1st, 7th and 14th days, add 10ml of STRESS ZYME per 10 U.S. gallons of aquarium water.
For existing aquariums: To maintain good water quality and healthy biological filtration, add 5ml of STRESS ZYME per 10 U.S. gallons of aquarium water weekly.
TIP: Continuous use of Stress Zyme will help keep the gravel bed clean and free of organics.
No refrigeration needed. Keep from freezing. Do not store above 120F"
But I don't see what the point of adding it to an already established tank would be.