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How Much Ammonia Is Toxic to Fish? (Ammonia Levels in Fish Tanks)

I am an aquarium enthusiast, and I particularly like offering advice to novice aquarists.

Monitoring Ammonia Levels

Most experts will tell you that ammonia is one of the compounds in the aquarium that will build to levels where it will kill fish. This is especially true when the aquarium is brand new and has not created an adequate population of beneficial bacteria to remove the ammonia immediately. It is true that ammonia can kill.

Ammonia Is Misunderstood

Ammonia may be the most misunderstood compound that occurs naturally in a functioning aquarium. It is created by most of the inhabitants in any body of water, and it is especially critical to understand for the beginning aquarist and his first tank.

Ammonia is a very popular compound to test in a new aquarium, but what should be done with those results is not always clear to the novice aquarist. Due to an apparently elevated level, in many cases, the various products offered for sale to control the danger in the new aquarium are not actually required. The resources spent on ammonia control might be better spent elsewhere.

Factors That Affect Toxicity

While it is true that ammonia can be very toxic when found in the aquarium, how deadly it actually becomes depends on independent water characteristics.

Ammonia and Ammonium

Quite often, it is left unsaid that there are actually two forms of ammonia: ammonia and ammonium, most often simultaneously tested, summed, and referred to as ammonia. Only one of them is actually toxic to fish!


More important to understand, ammonia toxicity is actually dependent on the pH of the water in which it is found. The more acidic the water, the less quantity of toxic ammonia is available in the water and the greater the concentration of non-toxic ionic ammonium.

Temperature (Possibly)

Another factor that is a possible contributor is temperature, but in the case of the aquarist, the temperature range is relatively small, from 60–80 F, and as such is not very critical in the overall results. I tend to completely ignore it, assuming the aquarium-specific test kit offers any conversions already correlated to standard tropical temperature ranges.

What Do Test Kits Really Show You?

Standard ammonia test kits (of whatever range) are normally only able to read the total quantity of ammonia components (ammonia + ammonium) in the water. The actual toxicity can only be determined with the use of a chart that correlates the actual amount of truly dangerous ammonia by referring to the actual pH of the water. Any ammonia reading is virtually worthless without correlating it to the pH reading of the aquarium water.

Where Does Ammonia Come From?

Ammonia is produced in the aquarium from quite a number of sources: respiration by-products, urination, and fecal matter as well. As soon as fish are added to a newly established aquarium system, ammonia is being produced as fish pass water over their gills to obtain oxygen and pass off many toxic compounds in the fish. Ammonia is one of these.

The Decay Process

The most dangerous source is often when anything is left over after the feeding process. As soon as it hits the bottom, it will start to decay. The decay process of any organic material will rapidly produce ammonia as a normal by-product.

The tragedy here is that this major potential source of ammonia is strictly under the control of the aquarist. These toxins are introduced by the person in charge, often without understanding that they can be the source of such a major problem. Proper feeding techniques are vital for the aquarium at all times, but they are of the utmost importance when a tank is becoming established and the system is still essentially sterile as far as ammonia reduction is concerned.

Dead or Dying Plant Matter

In the new aquarium with live plants, newly transplanted live plants will often die back somewhat due to transport stress and drop dead and dying leaves as they recover and create new growth. In many cases, the vegetation dies back significantly before it can establish new growth. These waste vegetation parts also add to the ammonia production as they decay. Just like excess food left after a standard feeding, all dead or dying plant matter should be removed as soon as it is observed.

Dead Fish

One of the greatest sources of sudden ammonia/ammonium surges in the aquarium, new or matured, is the death of a fish. The aquarium should be carefully observed daily to be sure all occupants are active and swimming. The best time for this inventory is during the feeding when all fish should be actively eating.

When a fish does expire, the body begins to decay almost immediately and rapidly becomes a huge source of organic material and heightened decay processes. The decay of a single body can quickly bring ammonia levels up to dangerous levels. These concentrations often overcome the natural balance and control exerted by beneficial bacteria that are available to eliminate it as fast as it is produced, at least in the mature aquarium.

Often the unobserved or late removal in the death of a single fish will end in the stressful poisoning of the rest in the tank. This single original source often starts a domino effect of death to all the other fish in the aquarium.

When Is Ammonia Actually Toxic to Fish?

Tested ammonia levels may appear to be quite high, but still do not affect the fish. The number of extra sales of ammonia control products resulting from this misunderstanding, either accidental or deliberate is massive. Just because the ammonia reading in a standard ammonia test kit is at a high level, does not mean the fish are actually in danger. This is because of the powerful interaction of ammonia compounds with pH levels.

Dangerous pH Levels

What is often left unsaid (or not understood) by the local resource trying to help reduce ammonia is that toxicity usually happens only if the pH is above 7.0. The lower (more acid) the pH level is, the more significant the quantity of ionic ammonium (NH4) and the less (toxic) gaseous ammonia (NH3) is in the water column solution. In this case, unless the concentration of ammonia is abnormally high, the fish are actually unaffected.

When the pH is high, a much greater amount of the ammonia determined by a test kit will be in the gaseous NH3 form and deadly to fish life. Toxic concentrations may cause loss of equilibrium, hyperexcitability, increased breathing, decrease in nitrogen excretion, not to mention death.

Using a Chart

Although a standard ammonia test kit can only show the combined amount of ammonia and ammonium present in the aquarium, the reading should be compared to a chart relating ammonia to pH levels. A reading of .05 mg/L of toxic ammonia (NH3) is at the very edge of safety for any aquarium. But that level can be reached when the total ammonia compounds are only 1.2 (NH3+NH4) mg/L if the pH is 8.0. However, it would take a concentration of combined ammonia of 7.3 mg/L to reach the same level of toxic ammonia in an aquarium with a pH of 7.2. Any pH reading of 7.2 and below will not have a problem with ammonia toxicity up to 7.3 mg/L total ammonia compounds or less.

Ammonia (NH3) Toxicity: Abbreviated Chart


8.6 0.22 0.44 0.68 0.90 1.12 1.34 Bold Underline = Toxic

8.2 0.10 0.20 0.30 0.40 0.50 0.60

7.8 0.04 0.08 0.12 0.16 0.20 0.23 Bold Italics = Marginal

7.4 0.02 0.03 0.05 0.06 0.08 0.09

7.0 0.01 0.01 0.02 0.02 0.03 0.04 Standard = Safe for Fish

6.6 0.00 0.00 0.01 0.01 0.01 0.01

6.2 0.00 0.00 0.00 0.00 0.01 0.01

1.2 2.4 3.7 4.9 6.1 7.3 NH3 + NH4 Combined (mg/L)

Ammonia Concentration in the Beginning Aquarium

In the ideal and textbook case, ammonia will gradually and naturally rise during the first ten days or so of the newly started aquarium, so it can be tested to monitor this rise on a daily basis. The fish are breathing and other decay processes progress much more rapidly than the initial population of the beneficial bacteria that reduce ammonia compounds to nitrite. The proper baseline of pH should also be taken and monitored at a less frequent interval.

Once the beneficial bacteria strains have established adequate populations through geometric replication, the ammonia level should suddenly and rapidly fall to zero. Depending on the amount of waste production and excess organics being added by the aquarist, this can take between 10 days and a few weeks. Much is dependent on the feeding regime. Once the sudden drop does occur, this indicates that the proper strains of beneficial nitrifying bacteria are now able to immediately reduce all the ammonia and ammonium that is produced in the aquarium from all the various sources.

Should You Use Chemicals or Supplements?

Although I am not a proponent of adding many chemicals to the water to perform changes in pH or other water characteristics, I do normally use a biological filter supplement to reduce the problems with maturing a biological filter. The main bacteria for ammonia reduction, nitrosomonas, is not the most robust strain, and can be overcome by predation and invasion of pathogenic species.

By inoculating the aquarium regularly, the nitrobacter and nitrosomonas populations have a chance to become dominant and become much better able to do the important jobs they have in the aquarium. The fish also seem much better adjusted in the entire process.

If testing subsequently indicates that the ammonia compounds are suddenly rising, then it is time to look for the cause of the problem.

Nitrite Concentrations

Although ammonia is often a deceiving reading for the novice aquarist, and in many cases no action needs to be taken, the same is not the case for the next step in the nitrogen cycle—nitrite. Nitrite is created by the beneficial nitrosomonas bacteria who use ammonia as an energy source; they use it as a food source and leave behind nitrite. Nitrite has no interdependence with any other water characteristics and is toxic to fish and other living things alike, no matter what. If it is concentrating, it is becoming more and more deadly. In the beginning, this is the compound that I believe should be monitored most closely.

Unlike ammonia, there really are not a lot of things that can be done to reduce or remove nitrite concentrations as they build, other than through dilution from regular water changes. Because there aren't a lot of other ways to eliminate nitrite commercially, it is not as focused upon as ammonia in the aquarium, even though, in my opinion, it is the cause of many more deaths than ammonia could ever be blamed upon. Besides dilution, supplementation of the nitrobacter bacterial strain that reduces nitrite is the only other viable way to make the water safer for fish during the vital maturation phase.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2009 blueram85


Stephen Tang on May 28, 2020:

How to join your group ?

Will on March 08, 2020:

Ammonia (NH3) does not get reduced to Nitrite (NO2-). NH3 is oxidized into NO2- by ammonia oxidizing bacteria and archaea because the oxidation number of N increases from -3 (NH3) to +5 (NO2-). Reduction refers to a decrease in the oxidation #. Similarly, NO2- is then oxidized into NO3- by nitrifying bacteria as the oxidation # increases from +5 (NO2-) to +7 (NO3-).

Bill on April 13, 2018:

Is the Fish tank Ammonia harmful to people? We live next door & smell of Ammonia is heavy when around people with Fish Tank...


Stuart from Santa Barbara, CA on August 04, 2016:

Great article, I am now subscribing. I am also a huge aquarium enthusiasts. I have 2 oscars and a jack dempsey, they are not full grown thank god because then I would need a bigger tank.

blueram85 on March 15, 2010:


You are right, ammonia is toxic, no matter what concentration or when it is evident. The problem is that the standard test kit measures two types of a related compound, the toxic ammonia and the non-toxic ammonium. As I mentioned on the page:

"There are actually two forms of ammonia - ammonia and ammonium, most often simultaneously tested and summed which is referred to as ammonia. Only one of them is actually toxic to fish! More important to understand, ammonia toxicity is actually dependent on the pH of the water in which it is found. The more acid the water, the less quantity of the toxic ammonia is available in the water and the greater the concentration of non-toxic ionic ammonium"

Whenever you have true ammonia in the tank, yes, it is toxic, but the overlooked problem is that often people assume the test kit is only returning ammonia readings - that is usually how it is sold, when in fact it is showing the sum of two compounds instead. Treating for ammonia when it is actually ammonium can cause chemicals to be added to the water that are not always healthy and don't need to be there. When pH is low, ammonium is what is actually there, and it is non-toxic - you don't have to worry about it and it will not hurt the fish. The compound produced when an ammonia compound is broken down, nitrite will still be produced, though, and that is toxic at all times. That is the one I usually worry more about.

flora on March 15, 2010:

i think that ammonia is really toxic to fish it does not matter if it has a little bit or much ph to be toxic for fish