Ammonia and Ammonium—What’s the Difference?

In our last article, we shed some light on ammonia and how it gallivants through your aquarium, wreaking havoc on your fish’s health. Now that your tank is cycled, any marauding ammonia molecules are immediately captured by an army of bacteria and sequestered away into less harmful compounds. But that’s not the end of the story! Did you know that ammonia has a mild-mannered alter ego? In freshwater aquariums, the majority of ammonia actually exists as its ionized form, ammonium.

Ammonium (NH4+) differs from ammonia (NH3) in a few ways. While ammonia is a dissolved gas, ammonium is an ion, similar to the Na+ or Cl- ions you would get from dissolving table salt. Ammonium is also a weak acid, while ammonia is a base. For fishkeeping purposes, ammonium is much less toxic because it cannot cross the cell membranes of animals. Ammonia can, and that’s why it’s so dangerous: any ammonia in the water can easily diffuse into the bodies of fish to wreak havoc on their internal organs.

In a typical tropical tank, there is a lot less ammonia than ammonium in the water—something like 1-2% ammonia compared to 98-99% ammonium. This ratio isn’t static! The equilibrium between ammonia and ammonium changes based on two factors: temperature and pH.

Higher temperatures make % ammonia go up (a little).

Higher pH makes % ammonia go up (a lot). 

The effect of pH and temperature on ammonia can be seen below in this table.


To read this table, simply find the number where your aquarium’s pH value and temperature intersect. So if your tank has a pH of 7.4 and a temperature of 24°C (75°F), 1.3% of your equilibrium is ammonia and the rest is ammonium.


Suppose you keep a Lake Tanganyika biotope full of colorful, alkaline-loving cichlids. You might be surprised to find that your tank with a ph of 8.4 contains about 11% ammonia. A pH increase of 1.0 caused a tenfold increase in ammonia concentration! This means that ammonia spikes are much deadlier in African cichlid tanks than typical setups, since at any given time, more of the nitrogen in your tank is going to be in the form of harmful ammonia instead of the milder ammonium. 

Those of us without African cichlids can still apply this knowledge when we use a liquid test kit. Most liquid tests (such as API’s master test kit) measure the aggregate sum of all ammonia and ammonium in the water without making a distinction between the two. By now, you can probably figure out why this could be very misleading—2.0 ppm that is 1% ammonia is a lot less scary than 2.0 ppm that is 10% ammonia!

Given the water tests below (“ammonia” refers to total ammonia/ammonium), which tank do you think is at higher risk?

If you guessed the second one, you are correct! High pH = more ammonia, so it is important to know your pH when using a liquid test kit to find your ammonia levels. 

I’m going to wrap up this post by discussing how all this ties into fish biology.

When a fish excretes ammonia out of its gills, how can it stop the freely membrane-hopping toxin from waltzing right back into the fish’s body cells? By turning it into ammonium, of course! 

Freshwater fish exploit the pH-dependent interchange between ammonia and ammonium to shut their waste products out of their bodies for good. When dissolved carbon dioxide is exhaled out of a fish’s gills, it forms carbonic acid, acidifying a thin layer of water around the gills. This acid envelope converts exiting ammonia into ammonium, which is unable to make its way back in! To me, this is a wonderfully elegant example of evolutionary “problem solving,” where the excretion of one waste product facilitates the elimination of another.

The Fabulous Ammonia-Ejecting Acid Envelope of Fish Gills