Ammonia Toxicity and the pH Relationship

By Tony Griffitts

Ammonia's toxicity to fish is very well know.  Most aquarium and pond related books usually dedicate at least a paragraph or two on the subject.  What is often not mention in many books is the relationship pH plays in the toxicity of the ammonia.

Ammonia concentration in a new aquarium or pond is a chemical we have to watch closely to make sure the levels do not reach a point where they start killing fish.  The death of many species of fish can start at as low as .6 parts per million (ppm).  In established systems the ammonia level normally reads 0 ppm.  When you test for ammonia with your aquarium or pond test kit, the reading you actually have is a combination of ammonium (NH₄⁺ or ionized ammonia) and ammonia (NH₃ or unionized ammonia) known as Total Ammonia Nitrogen (TAN).  Ammonia is the toxic part of the TAN.  Ammonium even at high concentrations does not cause mortality in fish.  Understanding the difference between two is crucial to figuring out how much toxic ammonia you really have in your system.  How much of the TAN you have that is toxic is greatly related to the pH of the water, and to a much lesser extent the temperature.  The higher the pH the greater amount of the TAN is ammonia.  Water with a temperature of 82° F (28° C), a pH of 7.0, and a TAN of 5 ppm has only .03 ppm ammonia.  If you are trying to keep Tanganyikan Cichlids in water with a pH of 9.0, that has a TAN of 5 ppm your ammonia level is 2.06 ppm (a deadly danger zone).  This is why saltwater fish and African cichlids are thought to be more sensitive to ammonia,  these fish are normally maintained in water with a pH of 8.2 or greater.  At a pH of 6.0, and 10 ppm of TAN, the ammonia is only .007 ppm.  While it looks like the fish mortality should be very high, the fish are doing fine.  The graph below provides a "True Free Ammonia" chart that can be referenced for figuring out how dangerous your TAN reading is.

Free ammonia is the toxic part of the Total Ammonia Nitrogen (TAN).  The ledger to the right provides the line colors for 1 ppm through 5 ppm, the left side of the graph is the "True Free Ammonia" reading based on the pH value on the bottom.  Notice that above the pH of 8.0 the toxicity of the TAN rapidly rises.

South American and West African cichlid breeders that maintain a low pH below 6.0 need to be cautious when performing water changes, as the low pH has an adverse affect on the nitrifying bacteria that converts ammonia to nitrite.  Because of the acidity these bacteria populations can drop so low that the TAN reading can rise quickly.  While the pH stays low the TAN reading is nearly all ammonium, but if you do a water change or add a alkalinity buffer to the system the ammonium can be quickly converted to ammonia, potentially causing ammonia poisoning.

Ammonias toxicity is greatly affected by the pH in the system.  You need to test both the pH and the total ammonia nitrogen level to find out the true ammonia toxicity level.  The TAN is far more toxic at higher pHs and than at lower pHs.