Forms of ammonia

• Un-ionized ammonia (NH3): This is the toxic form of ammonia. It is more toxic at higher temperatures and pH levels.  
• Ionized ammonia (NH4+): This is the less toxic form of ammonia. It is more toxic at lower temperatures and pH levels.  

The total ammonia nitrogen (TAN) level in water is the sum of both un-ionized and ionized ammonia.