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Aquarium Ammonia Levels Explained (With A Simple Chart)

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Ammonia is bad for fish tanks; we all know it. But there is a wide range of ammonia levels, and each one affects the fish differently. As this topic has preoccupied me a lot in the past, I decided to write an entire article about it.

Here is a simple chart that shows how toxic the different levels of ammonia are:

As we move forward, I will elaborate on each toxicity level, including 0.25, 0.50, 1.0, 2.0, and 4.0 ppm. Then, I will discuss what can cause low levels to be toxic as well and what steps you should take if your fish are suffering.

Aquarium Ammonia Levels Explained

The nitrogen cycle is a vital component of a healthy aquarium because it allows nitrifying bacteria to turn ammonia into nitrite and then nitrate.[1]

You have to cycle the tank to introduce these nitrifying bacteria. In some cases, it included adding ammonia to a new tank.

However, the presence of nitrifying bacteria cannot prevent the ammonia concentration from rising.

This is because ammonia comes from decomposing organic matter such as leftovers and waste. Therefore, you cannot keep ammonia out of the aquarium, not permanently.

But when does ammonia become dangerous? At what level does the substance become toxic to fish?

For those of you who are in a rush, here is a simple table that gathers the essential thresholds and their impact on aquarium fish:

0.00 ppmSafe
0.25 ppmMildly Toxic
0.50 ppmToxic
1.00 ppmDeadly
2.00 ppmDeadly
4.00 ppmDeadly

0.0 ppm

You should keep ammonia levels at 0.0. This is the ideal concentration.[2] At 0.0, the ammonia is not necessarily absent. Rather, the substance is simply undetectable.

Standard testing kits will check the combined total of ammonia and ammonium in the water. You cannot stop cycling a tank until the ammonia levels are undetectable by a standard testing kit.

People treat high ammonia levels as a death sentence for fish, but that is not always the case. Fish can survive ammonia levels above 0.0 ppm.

However, the presence of ammonia speaks volumes about the health of the tank. An aquatic environment with 0.0 ppm has a healthy bacteria colony.[3]

On the other hand, even though the ammonia is unlikely to kill the fish at 0.049 ppm, those levels should concern you because of what they have to say about the population of nitrifying bacteria and the presence of decomposing organic matter.

0.25 ppm

0.25 ppm is a safe level for a cycling tank that has fish. Fish are better off with 0.0 ppm. But if you have to expose them to ammonia, 0.25 ppm is somewhat acceptable.

But that is only true in the short term. Long-term exposure to this level of ammonia can compromise the creature’s health. Established tanks with 0.25 ppm of ammonia require a water change.

Experts encourage water changes in this case because the ammonia concentration can easily exceed 0.25 ppm if it goes unchecked. You are better off pushing the concentration back to 0.0 ppm.

0.5 ppm

0.5 ppm is as far as you can permit ammonia levels to rise in a tank with living organisms.[4] While aquarists typically endeavor to keep the ammonia levels at 0.25 or less, fish can tolerate 0.5 ppm.

Although this concentration will induce significant stress.[5] Even though 0.5 ppm can kill the aquarium’s inhabitants in the long run, most aquarists won’t panic until the ammonia concentration exceeds 0.5 ppm.

At 0.5 ppm, you have to perform a water change. Adjust the feeding habits, reducing the size and frequency of meals. You should also siphon the substrate and clean the filter.

1.0 ppm

Some people think that every ammonia concentration higher than 0.0 ppm is dangerous. But that is not necessarily true. Ammonia levels do not become truly dangerous until they exceed 0.5 ppm.[6]

1.0 ppm is way higher than 0.50 ppm. Therefore, it should concern you. Ammonia concentrations this high will kill ordinary fish or compromise the immunity of their hardy counterparts.[7]

Therefore, you have to take immediate action. Clean the tank. Remove pollutants like dead plants and animals. Check the filter for debris and unclog it where necessary.

You should also perform a significant water change. Aim for 50 to 90 percent, depending on the health of your fish. Where necessary, add some water conditioners. They will neutralize the ammonia.

2.0 ppm

2.0 ppm is too high. These ammonia levels will either kill the tank’s inhabitants or cause cellular damage whose consequences may only manifest in the long term.

Perform a water change of 50 percent or more. You can also add water conditioners if the fish are too weak to survive a significant water change.

While the fish recover, you should take steps to remove any debris that has contributed to the high ammonia concentration.

4.0 ppm

If 2.0 ppm is bad, 4.0 ppm is worse. Stop feeding the fish and perform a water change. If the fish are too weak, use a water conditioner to provide relief.

Transfer the tank’s inhabitants to a second aquarium and perform a significant water change where possible.

Can Low Ammonia Levels Still Be Toxic?

Identifying safe ammonia levels is not quite as straightforward as people think. On paper, 0.25 ppm is clearly safer than 4.0 ppm.

However, if your fish keep dying even though the ammonia levels are not exceptionally high, you probably forgot to take these three factors into account:

1. Total Ammonia Nitrogen (TAN)

Testing kits are designed to measure the Total Ammonia Nitrogen, which is a combination of ammonium ions and non-ionized ammonia gas.[8] While the non-ionized gas is toxic, the ammonium ions are mostly harmless.

Therefore, it isn’t enough to determine the Total Ammonia Nitrogen (ammonium ions + non-ionized ammonia gas).

You must also determine the fraction of non-ionized ammonia in the TAN to better understand the ammonia’s toxicity in your aquarium.

2. The Water Alkalinity

The pH affects the toxicity of ammonia. If you look at the chart Ruth Francis-Floyd from the University of Florida has published (Ammonia in Aquatic Systems), you will notice that it doesn’t stop at revealing the toxicity of ammonia at different levels.[9]

The table also reveals the toxicity of ammonia at each pH level, proving that the pH can affect the toxicity of ammonia. A higher pH tends to produce more significant ammonia toxicity.

For instance, ammonia toxicity is worse at a pH of 8.0 than it is at a pH of 7.0.[10] When you measure the ammonia concentration, you should also test the pH. This allows you to identify ammonia’s toxicity with greater accuracy using a chart. 

3. High vs. Low Temperatures

The temperature can affect ammonia’s toxicity, but only to a small extent. Ammonia’s toxicity will drop as the pH and temperature fall. This is why some ammonia toxicity charts include the ammonia on one axis and the temperature on another.

But the temperature is not significant in the grand scheme of things. The variable only matters within the 60 to 80F range, and even then, many people ignore it.[11]

Accounting for the temperature allows aquarists to estimate the ammonia toxicity more accurately, but most people can survive comfortably without this factor.

How Do I Bring The Ammonia Down To Zero?

If you’ve measured levels of 0.25 ppm or higher, I suggest fixing it. You shouldn’t wait for the ammonia to spike or for your fish to get used to it.

Dealing with ammonia is divided into two. First, you should neutralize the ammonia. Then, you should eliminate factors that will make it spike once again.

To neutralize this toxin, I personally recommend getting the API AMMO-LOCK (link to Amazon). It will instantly detoxify the ammonia in your tank.

Simply add 5 ml per 10 gallons. Repeat this every two days until the ammonia is no longer detected. As for the test kit, I use the well-known API FRESHWATER MASTER TEST KIT (link to Amazon).

Then, eliminate factors that produce ammonia. Fish tanks get ammonia in many ways, including fish waste, rotten food, and dead organisms.

That is why cleaning the tank is crucial. Siphon the substrate and count your fish. Make sure no dead organisms hide between your decorations.

Try not to touch the filter’s media. This part usually contains nitrifying bacteria that turn ammonia into nitrite. If you want to clean it, wash it with the aquarium water.

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Ammonia can be tricky. In some cases, fish will tolerate levels of 0.50 ppm but will suffer at 0.25 ppm. Generally, it depends on your tank and your fish.

As a rule of thumb, you should aim for 0 ppm. Even if your fish are used to higher levels, they will be stressed and probably won’t live as long as they should.

If the temperature isn’t suitable and the pH is too high, the ammonia will become more toxic as the non-ionized gas increases. That is why 0.25 can be more harmful than 0.50, for example.

Ensure you test the water weekly and that your heater is functioning properly. I also recommend getting a detoxifier, even if the ammonia levels are at zero. That might become handy in the future.