7/8/11 Edit - Stickied this to keep the information at the top for the new folks.

We all know what the Nitrogen Cycle is (for those of you who don't, click here; take notes!), but rarely does anyone explain why the components of the nitrogen cycle (ammonia/nitrite/nitrate) are harmful to fish. The ongoing and persistent explanation is "Ammonia and nitrite bad, nitrate less bad, because we said it is", and while this certainly is true, I thought it might help some of the newer folk to explain exactly what each does to your fish. I'll try to keep it as simple as I can while still presenting all the information needed; I know a lot of you probably don't want to learn biochemistry just to keep an aquarium, but this is important stuff.

Ammonia (NH₃)

To start off, what is this ammonia business we keep hearing about? Ammonia is a compound of nitrogen and hydrogen. Although ammonia often gets a bad rap through our hobby, it's actually a critical part of our ecosystem, as it forms an essential link in the food chain by being food for plants, which feed the cows, which feed the people...you get the point. That's not to say that ammonia isn't bad stuff, however. It's caustic, meaning it has the ability to corrode matter; you can see how this can be a problem for living organisms. In humans and other terrestrial animals, ammonia is converted into a phosphate by an enzyme in our body, and is then put into our urea cycle to either by recycled by the body or excreted in our urine.

Aquatic animals like fish and amphibians however do not possess a biological method for converting ammonia, and thus eliminate it by direct excretion. This is mostly done through the gills for fish, and through the skin for amphibians. As ammonia is constantly being created in their bodies through a process called amino acid deamination, you can safely assume that your lovable little fishies are swimming ammonia factories. In nature the majority of this is diluted by the sheer amount of water they swim in; in captivity however, the volume of water most fish are kept in prohibits this from happening.

How does ammonia harm our fish?
Ammonia will directly harm your fish by burning the sensitive tissues of their gills. Just as exposure to gaseous ammonia will harm our lungs (at 35ppm atmospheric concentration, it can cause irreversible health problems in as little as 15 minutes of exposure), exposure to ammonia in aquarium water can harm your fish's gills. This has the direct and immediate result of hindering respiration, but also the long term inhibition of the gills to perform an acid-base balance (keeping the pH of the fish's blood at the proper level for biological functions to perform properly).

Ammonia exposure can also cause physical damage to internal organs, skin, and fins, immune system suppression, cause stunted growth, nervous system damage, and a whole host of other issues. Suffice to say, ammonia is never something you want in your aquarium in ANY quantity.

Signs of ammonia poisoning

• Gasping
• Sudden death of stock
• Twitching, whirling, or other abnormal swimming
• An excess of body mucus
• Bloody patches on the body and fins

Nitrite ( NO₂⁻)

The second stage in the nitrogen cycle, Nitrite is no less dangerous than ammonia, but for different reasons. An ester of nitrous acid, nitrite has a nasty little habit of bonding with red blood cells and preventing them from moving oxygen throughout the body. This causes a condition known as hypoxemia, and can cause organ failure, nervous system damage, and brain damage. As a fish's gills essentially put their blood right out in the open, you can see how this can negatively effect their well-being in a short amount of time.

Imagine taking a breath, and every breath after that you grow progressively more and more tired. No matter how much you take in oxygen, you're always out of breath, until you die from organ failure. Not fun.

Symptoms of nitrite poisoning

• Heavy, labored, or otherwise impaired breathing
• Gasping at the surface or staying near oxygen-rich areas of the aquarium (air stones, intake tubes, etc.)
• Brown gill tissues

Nitrate (NO-3)
Nitrate is the least harmful of the three, but it can still have fatal consequences. 95% of the aquariums I test have at least a little bit of nitrates in them, and it's usually an unavoidable problem in most non-planted freshwater aquariums. While most people consider nitrate harmless except in larger quantities, the truth is that even in concentrations higher than 30ppm nitrate can cause an inhibition of growth and development, cause nervous system damage and degradation (through the inhibition of vitamin B12 uptake, a crucial part of proper nervous system function), and cause general overall stress for aquatic animals. Nitrate has secondary ramifications in that it can be used as a food for algae, which in epidemic proportions can cause major issues with water quality and chemistry. Algal blooms can also lead to water anoxia (a lack of oxygen in the water).

Unlike ammonia and nitrite, nitrate is not converted in most home freshwater aquarium systems into another substance. The most reliable (and easiest) way to lower nitrate levels in your aquarium is to do a simple water change. Chemical filtration methods are available to strip nitrates from the water, but these don't revitalize the ion balance of the water like a water change will.

Symptoms of nitrate poisoning

Nitrate poisoning is a little sneakier than ammonia or nitrite poisoning, as outward symptoms may not present themselves until the latter stages of the poisoning process. General listlessness, lack of feeding reflex, erratic swimming and behaviors, and overall poor health are all signs of nitrate poisoning; generally if these are noticed, your fish has suffered at least a little permanent damage. Your only recourse is to quickly rectify the nitrate issue and hope for the best.

Here's the good news: unless you are cycling a new tank, you won't generally see the first two unless something drastic has gone wrong with your aquarium. Having a good set of test kits on hand capable of testing these three compounds (in addition to pH, hardness, and alkalinity) is never a bad idea, and a water test should always be your first course of action when presented with an unknown problem. Having accurate numbers to present to us here also helps us help you. Learn the nitrogen cycle, learn how to properly maintain your tank, and with any luck The Big Three should never rear their ugly heads past the cycle process.

Cheers!

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Dont forget the smells of the nitrogen cycle.
You thought poo smelled bad......

excellent article/post.

Always learn stuff with these types of posts.


One correction in that ammonia is not a precurser of plant food but rather is a plant food directly. Plants will actually prefer and consume ammonia vrs nitrates. But in mature aquariums there is enough aerobic bacteria are reducing the ammonia so the plants have to use nitrates for their nitrogen.

Other then that good article/post

my .02

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fw leiden since 1979, fo salt since 1979, mixed reef 55g 2002-2009. Strong emphasis on the tank taking care of itself. Balanced with plant life, no water changes, tap water, no filters in FW. Only dosing calcium, alk, mag in marine reef tanks. http://www.aquariumforum.com/f15/my-...ods-26410.html
recent tanks (till 2009) 7 years- 10g FW leiden 7 yrs, 55g mixed reef 7, 2 yrs, 20g FW leiden, 10 g fw leiden , 29g mixed reef, current tank 55g leiden

Corrected, and thank you. I knew I was trying to say something specific originally, but a shiny object took my attention and I typed up something wrong.

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Just outta curiosity what to you hear about the toxicity of nitrItes and higher pH? I have had marine fish live for weeks at levels the pegged the test kit (5ppm+). that was before I stopped adding food so the nitrItes spike passed.

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fw leiden since 1979, fo salt since 1979, mixed reef 55g 2002-2009. Strong emphasis on the tank taking care of itself. Balanced with plant life, no water changes, tap water, no filters in FW. Only dosing calcium, alk, mag in marine reef tanks. http://www.aquariumforum.com/f15/my-...ods-26410.html
recent tanks (till 2009) 7 years- 10g FW leiden 7 yrs, 55g mixed reef 7, 2 yrs, 20g FW leiden, 10 g fw leiden , 29g mixed reef, current tank 55g leiden

Nitrite bonds with red blood cells just as readily at high pH as it does at low pH. Ammonia is the one you need to be careful of when it comes to high pH. A jump in pH from 7.0 to 8.0 increases the amount of toxic ammonia in a tank ten-fold. Higher temperatures play a factor in this as well.

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Good discussion.

Bumping back to the top of page one for the new folk.

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Ttt.

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Question for you then - what happens with ammonium? (NH4). The article I've always toted about the nitrogen cycle has said some of the fish waste turns into ammonium, and some into ammonia, depending on your pH levels (pH<7 = ammonium, pH>7 = ammonia). The ammonia is toxic, but the ammonium is not. Do plants benefit from ammonium? And if they don't consume it, I'm assuming the only way to remove ammonium is with a PWC, correct? And also, do the ammonia test kits factor in for this potential ammonium concentration?

My source:
The Aquarium Nitrogen Cycle

Great article by the way! +5

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Officially fishless until after the move....tune in later for details!

long time ago I heard that ammonium->ammonia->nitrIte->nitrates-> plants or anaerobic bacteria->plant tissue or nitrogen gas, was a more complete nitrogen cycle.

the important thing here was the ammonium->ammonia part.

but that's been a long time ago. Hard to remember these things.

especially since I don't measure any ammonia in my tank anyway.

my .02

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fw leiden since 1979, fo salt since 1979, mixed reef 55g 2002-2009. Strong emphasis on the tank taking care of itself. Balanced with plant life, no water changes, tap water, no filters in FW. Only dosing calcium, alk, mag in marine reef tanks. http://www.aquariumforum.com/f15/my-...ods-26410.html
recent tanks (till 2009) 7 years- 10g FW leiden 7 yrs, 55g mixed reef 7, 2 yrs, 20g FW leiden, 10 g fw leiden , 29g mixed reef, current tank 55g leiden

Ammonium is ten times more toxic than ammonia, if I remember correctly. 95% of the people with freshwater aquariums will never encounter ammonium in anything more than trace levels, since you don't see it below a pH of 8.0 or higher. It's much more of a concern for marine aquariums, which are constantly above that pH (or should be).

The only freshwater aquariums that would have a reasonable chance of experiencing death due to ammonium are african cichlid aquariums that are being kept at a higher pH for better fish health, and even then ammonium is converted at a high enough rate that it's unlikely it would cause any damage in a well-established tank.

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Just another bump for the new folk!

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Bump to bring this back to the top, hate to see this info get buried.

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This is amazingly useful, thanks for writing it up!

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Just starting:

10 Gallon
1 Honey gourami
3 Peppered corys
3 Ghost shrimp

Wisteria
Aponogeton
Amazon swords
Anubias nana
Marimo moss ball
rotala indica

Why not make this a sticky. Very good mostly scientific data. I learned something and I've been at this awhile.

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29g Tall FW: Many Swordtails: 1 lemon, 6-8 neon/pineapple, 1 Gold Tuxedo, ? fry; 1 Clown Pleco; 3 Otocinclus Cats, and MTS.
Planted: Amazon Swords.

The only place where success comes before work
is in the dictionary.

My ammonia and nitrites are always 0 and nitrates 0-5, still lots of pleco poop. I usually just leave it and let it dissolve on it's own.