What Is Ammonia And Ammonium In An Aquarium?

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What Is Ammonia?

Everything produces ammonia (NH3) in the form of waste. But, in the aquarium, plant material, dead fish, and uneaten food rotting away add to the waste already produced. Ammonia can quickly become toxic to fish and burn their gills, affecting their respiratory system. It can also compromise their immune system, lead to secondary diseases and infections, burn their skin and gills, stunt their growth, and kill them in high doses.

If you’re asking yourself, “Just how much is toxic?” that depends on other parameters in your water and the fish that you keep. At a higher pH and temperature, ammonia becomes more toxic at a much lower ppm. But at a lower pH and temperature, it takes more ammonia to become toxic. The more unmetabolized ammonia in the water, the higher the pH. As bacteria metabolize ammonia, the pH lowers, making the remaining ammonia slightly less toxic – but not by much. At particularly low pH levels, ammonia becomes ammonium (NH4+).

Water test kits usually measure the total of both NH3 and NH4+, so the toxicity is hard to gauge with a test kit alone. pH, temperature, and species also play a key role in the toxicity “threshold.”

Ammonium Versus Ammonia

NH3 is the unionized form of ammonia and is more toxic. NH4+ is the ionized form (ammonium) and is far less toxic. Studies indicate it’s at least 100x less toxic than the unionized form. Additionally, the ionized form can raise the pH of your aquarium while the other doesn’t. Why is that? This is going to get a little science nerdy, so roll up your sleeves and buckle up.

When NH3 combines with H2O (water), it creates one NH4+ and one OH- ion – two ions total. NH4+ being ammonium and OH- being hydroxyl. When there’s more hydroxyl ions in the water, the pH of the water increases. However, when NH4+ is combined with OH-, it produces NH3 and the pH drops due to fewer free hydroxyl ions. Wait. What?

Yeah, here’s the thing: there are always two forms present in your aquarium – NH3 and NH4+. How many of each depends on your starting pH. At a lower pH, NH3 is produced by your fish, interacts with the water to form NH4+, and OH- (hydroxyl ion.) The free hydroxyl is usually grabbed by another acid before it can attach back with the NH4+ to form NH3 again. At a higher pH, there’s usually more OH- than NH4+ and it’s possible that any NH4+ might not stay that way for more than a few seconds.

NH4+ cannot be taken up by plants in large doses without potentially killing them, NH3, however, can be taken up by plants just fine. At a particularly low pH, all NH3 becomes NH4+ and at even lower pH, microbes are unable to gain enough of a foothold to effectively oxidize it into nitrite and, later, nitrate. This is why when your pH is particularly low, you’re unable to cycle your aquarium – there’s nothing to cycle.

Examples of Ammonia

It’s a chemical itself, usually sold as a household cleaner, and measured in parts per million (ppm) in the aquarium. A healthy tank will fall somewhere between .0 and .25ppm. Anything above .50ppm usually starts to become toxic for all fish and will begin affecting their gills and other systems.

Of course, ammonia can be removed with a water change and usually some water additives (although the latter are short-term fixes.) Proper filtration can also help keep ammonia at bay, but to understand that, you have to understand the nitrogen cycle that happens in your tank.

Ammonia in high levels for extended periods can cause ammonia burn, such as in the image above.