API QUICK START nitrifying bacteria allows for the instant addition of fish, as it immediately starts the natural aquarium cycle with beneficial bacteria, which converts toxic ammonia into nitrite, then into harmless nitrate to help prevent fish loss in your tank. Best for use when starting a new aquarium, after water changes and filter changes, and when adding new fish to an existing aquarium. API QUICK START nitrifying bacteria may be used in both fresh and saltwater aquariums.