One and Only Live Nitrifying Bacteria removes toxic ammonia and nitrite, instantly working to prevent new tank syndrome by establishing a biological filter in your newly set-up aquarium. Use when setting up a new tank, after a disease treatment or water changes. 4 oz size for freshwater aquaria up to 60 gallons. One and Only has no sulfur or other offensive smells. Made by the recognized expert in aquarium nitrification and microbiology.