Ammonia Test Result is More than Ammonia

Published 20211006

The ammonia test result is the most misunderstood water chemistry test aquarium and pond hobbyist perform. It is a much more complex test that does not stop with the result in the vial. The ammonia test result is only the first step in a four-step procedure.

First, you must understand what the ammonia test kit is actually measuring. If you take a look at the "Freshwater Ammonia" card in the above photo you will see "(NH₃/NH₄⁺) COLOR." The ammonia test kit result gives a reading that is actually a combination of two chemicals NH₃ (un-ionized ammonia) and NH₄⁺ (ionized ammonia, also known as ammonium) which make up the total ammonia nitrogen or TAN. A color other than yellow only means that your result is positive and the chart numbers are for TAN, not just ammonia.

The two chemicals that make up the TAN, ammonia (NH₃) and ammonium (NH₄⁺) are very different even though the name looks similar. Ammonia concentration of as little as .6 ppm is toxic to many species of fish. If fish are exposed to .6 parts per million (ppm) of ammonia for 24 hours or more some mortality would very likely be the result. Ammonium (NH₄⁺) on the other hand is not toxic to fish in the concentrations we see in our systems.

So how do we know how much of the TAN is ammonia? The next step is to test the pH and check the temperature. The ratio of ammonia and ammonium is greatly affected by pH and to a lesser extent temperature. At a pH of 7 or below almost all of the result is ammonium. Once you have collected these three data points you can enter them in the online "Free Ammonia Part of TAN Calculator" below. If your temperature reading is in Fahrenheit you will first need to convert it to Celsius which a "Temperature Conversion" field is provided.

Before we continue, first let's put to bed one of the many common aquarium myths:

"A positive ammonia result means a system is not cycled."

A positive ammonia/ammonium result does not always mean a system is not cycled. On a well-established system, it is possible to have a positive ammonia/ammonium test result when the pH is 6.5 or lower. At a pH of 6.5, the nitrogen cycle is degraded and at 6 it stops. It is common to see a positive ammonia/ammonium test result for hobbyists keeping fish in very low carbonate hardness (KH) water with high nitrate. Nitrate is an acid and when KH is low it can quickly drive down the pH. The pH test results in the photo above are from a system that has been set up for months with a KH less than 1 dKH (1 degree of carbonate hardness equals 17.86 ppm) and a nitrate level above 160 ppm. The nitrite on the system is 0 ppm.

After a filter cleaning or overly aggressive substrate vacuum, it is possible to have an ammonia and/or nitrite spike a few days later due to the loss of too much nitrifying bacteria.

You may have had or have seen others with a system with an ammonia test result that appears to be off the chart but the fish appear to be acting normal. This is because the ammonia test result is almost all ammonium, not ammonia. Checking the pH will confirm this is the case with the result below 6.5.

Water changes on systems with a pH below 6.5 and a positive ammonia/ammonium test result can be a little tricky. Water changes reduce the amount of nitrate (an acid) and often increase the amount of KH. This will raise the pH and convert some of the remaining ammonium to ammonia. If the system has not had a water change in months it is also possible to cause pH and/or osmotic shock to the fish. It would be best to do small 25% to 50% water changes over the course of several days to bring the nitrate down and slowly raise the pH. It may take some time for the beneficial bacteria population to catch up with the new ammonia in the system so you will need to monitor. The system may need to re-establish a healthy nitrogen cycle which could take a few days or weeks depending on how long the system was below a pH of 6.5 and the bioload. After a water change, you must monitor the ammonia/ammonium and nitrite for several days to make sure they do not reach a toxic level (24h .6 ppm ammonia, 2 ppm nitrite).

Running a system that is intentionally kept acidic for the purpose of breeding soft acidic water species has its own challenges. The lack of minerals and low pH are inhospitable to nitrifying beneficial bacteria. But that does not mean it is bad for the fish. The ammonia that the fish excrete through their gills is quickly converted to ammonium and becomes non-toxic. Since there are little or no nitrifying bacteria to process the trace amount of ammonia the nitrite remains at 0 ppm. Water changes should be done with water that has been pre-conditioned (KH removed and filtered through peat) so it is acidic before adding to the system. If at some point you want to raise the pH to the basic (alkaline) side you should expect to have to go through the normal time it takes to develop a mature nitrogen cycle.

Maintaining a healthy mature nitrogen cycle may require the hobbyist to adjust the water chemistry to maintain a pH that supports nitrifying bacteria colonies.

Remember the ammonia/ammonium test result only shows if the result is positive. pH and temperature determine how much of a positive result is ammonia. Finding out how much ammonia is in a test result is a four-step process:

1. Ammonia test results = total ammonia nitrogen (TAN/ammonia and ammonium)
2. pH
3. Temperature
4. Enter values in the online calculator