The Nitrogen Cycle

[Ardan]

The Nitrogen Cycle

In an aquarium, there are waste products given off by the fish, or by uneaten food or other organics such as plant decay. All contain Nitrogen. Fish give off waste products, including ammonia through the gills. Uneaten food and solid wastes are broken down into ammonia by fungi and many bacteria that we refer to as heterotrophic bacteria.
The ammonia can be in two forms. Toxic (to the fish) ammonia is in the form of NH3 and occurs when the ph is greater than 7.0. Nontoxic ammonia is NH4 and occurs when the ph is below 7.0. Ammonia is very toxic to fish.
Ammonia is broken down into “Nitrites” (NO2) by special bacteria called “Nitrosomonas” bacteria.
Then the Nitrites are broken down into “Nitrates” (NO3) by another special bacterium called “Nitrobacter bacteria”. Nitrates are not harmful to fish unless they become high. The most effective way to keep nitrates low is through water changes.
In a new aquarium very little of the nitrosomonas or nitrobacter bacteria are present, and need time to reproduce to sufficient levels to breakdown their respective nitrogen compounds. The bacteria attach to surfaces in the aquarium such as gravel (if you have it) or the material in the filter, such as sponges that have a large surface area for the bacteria to attach to.
Ammonia levels can start to stress the fish at levels of more than .3 mg/l depending on the ph and the temperature of the water. (Untergasser 1989) To minimize the stress the ph level can be kept below 7.0 and /or water changes can be used to keep the ammonia level below .3 mg/l. Usually the easiest approach is to do water changes. High levels of ammonia may show up on the fish as hemorrhages on the gills. The gills may appear “lilac” in color. (Untergasser 1989)
Nitrite in the water is absorbed by the blood of the fish. This reduces the bloods ability to transport oxygen to the fish’s body. Continuous exposure to nitrite can lead to “brown blood disease”, where the hemoglobin in the blood is bound to nitrite and the fish suffocates from lack of oxygen. Non iodized salt can be added to the water at 2 tbsp/10 gallons water, to help the fish cope with nitrite. Water changes are one of the best ways to cope with high nitrite levels.
When ammonia and nitrite levels reach zero, we refer to the aquarium as being “cycled”. This is having enough of the proper bacteria in the aquarium to convert the organics into nitrate. This usually takes from 4 to 6 weeks. Speeding up the cycle can be achieved through addition of gravel or filter media from an already cycled aquarium or by the use of *bacteria products sold at a pet shop.
Sources
Handbook of Fish Diseases by Dieter Untergasser, TFH Publications, Inc 1989

Maintaining a Healthy Aquarium by Dr. Neville Carrington, Salamander Books 1985

Internet resources


http://faq.thekrib.com/begin-cycling.html
This site has a good chart on cycling time and effects of ph and temp on ammonia toxicity.



hth
Ardan

[Carol_Roberts]

Hi Ardan:sunshine:

This is one of the best articles you've written and one I hope everyone reads.  It clearly states the nitrogen cycle - the most important process in the aquarium!

Please put a sticky on it so future memebers can find it easily.

Carol :heart1:

[Ardan]

Thank you for the kind words Carol!
I hope it helps people understand the nitrogen cycle a bit.

Thank you very much!
Ardan :sunshine:

[cobalt]

Well typed steelhead do you publish science alot?
if you are interested in more data on the bacteria in the cycle Tim Heanovac From Tropical Fish Mag. Did his Phd on the topic and i saw an article about it.
I think he did a bunch of PCR's on the typical flora of the filtrers.
Cobalt

[Ardan]

Hi Cobalt,
 Thank you for the kind words. :) Actually I'm a welder/metal fabricator. The only writing I do is here at Simply. I am just trying to help people understand the hobby better and I like the sciences of this hobby, so it is interesting to me to research and share the information.
I will have to look for that issue of "Tropical Fish Magazine". Is that the June issue?
Thank you very much!
Ardan :sunshine:

Ardan,

     Just finished reading your thread. OUTSTANDING!!!
Lots of real good and useful information.

Miles

[brewmaster15]

Hi Ardan ,  
 Very Nice write-up! and references. They get better and better!

Hi cobolt,
I have read alot of Dr.Hovenecs papers. He works for Marine land.  There area good number of his papers  at that site...

http://www.marineland.com/drtims_reports.html
http://www.marineland.com/reports/13Nitrospira.html
  His research is interesting in that  it  indicates  that what we all have taken to be Nitrosomonas and nitrobacter in our aquariums may be a totally  different bacteria all together....Nitrospira spp.
  ...Thats science  for you as soon as you think you have all the answers ,  you realize  you need to revise  you understandings! ;D

take care,
al

[Discus_Lover]

Ardan,

As usual, you are selecting a very helpful article. It's good to be kept as reference.
There is only one point which I would like you to clear it up for me:
Those useful bacteria 'Nitrosomonas & Nitrobacter' are very slow in replication and it takes them around 4-6 weeks to catch up with nitrite production.
The question is, daily 30% water change will not slow the replication of those beneficial bacteria beside the difficulty the Discus might face in adaptation with the new water. Most of common books for fresh water fish keeping recommends W.C. weekly 10% or bi-weekly 20%?. Is this rule dose not aplly for Discus.
???

[Ardan]

Hi,
Daily wc may keep the ammonia and nitrite levels low, thus slowing the replication of the nitrosomonas and nitrobacter bacteria. But it is still best for the discus to keep the ammonia and nitrite low.
This is one of the reasons I like a fishless cycle of the aquarium before adding fish, so that the fish and I do not have to deal with ammonia/nitrite stress.
Daily wc are best for discus IMO. I can see a great improvement in my discus by doing wc.

hth
Ardan :sun:

[Karl]

Very useful summary on nitrogen cycle, Ardan. Thanks!

When I first noticed Untergasser's article mentioning ammoina was in an non-toxic form with PH below 7, I started wondering whether it implied our tank was free of ammonia problem if we keep the PH below 7. So no need to test the PH level then?

Karl ???

[Serpae]

Hi guys and gals, not sure here, but isn't NH4 ammonium, not ammonia NH3, I stand corrected on this one.

[Karl]

Oh yes, you are right. NH4+is ammonium.

Karl

[Serpae]

Optimum pH for sustainable biological activitiy (microflora) in the aquarium is stated to lie between pH 6.5 and 8.0. Bacteria tolerate higher temperature's (30C) and lower Ph value's (down to 4.5) than they do with the same Ph values but at a lower temperature (15C). Bacteria in Ph values of lower than 3.5 regardless of temperature virtually cease to exist.
"Why isn't water just water anymore" ???

[jim_shedden]

Ardan : Thankyou ........plain and simple.

Jim

[vinson]

Ardan

Great stuff. Thank you. :)

[SnowCichlid]

Yes teh nitrate system is so important.... I hate wehn you find people posting saying that they dont or barly do water changes.....and they wonder why their fish dont live that well

[Luca]

Hi Arden, just one question:
you mentioned that Ammonia is toxic to fish at ph over 7. However earlier on this forum I read that it is better not to play with the ph and leave it at stable tap water range at around 7.6-7.8

So should i aim to keep it at below 7 or do regular w/c's keep the Ammonia levels at bay?

Luca.

[Carol_Roberts]

With a cycled filter and daily water changes you shouldn't have a problem with ammonia in your tank.

[Ardan]

Hi,
Its best not to mess with the ph unless a must. Stability is important.

However if using a med, sometimes water changes are not recommended (using antibiotics for bacteria in early stages of treatment), In this case (Or at least I have done it) ph can be lowered to offset the buildup of ammonia.

Also it is just good to know that at higher ph , ammonia is more toxic. so that if you are dealing with ammonia and have high ph water, that you make more water changes to keep ammonia at 0.

One thing to keep in mind, Nitrite is more toxic if ph is below 7. so once the cycle goes into the 2nd phase (ammonia to nitrite), the lower ph would not be recommended.

One other note, lower ph = slower bacteria growth, this can be useful when treating bacterial infections, however the good bacteria for the filter also grow slower.

hth

[Luca]

Ok thanks very much for the feedback guys. I know it's too soon for me to be thinking about breeding, but when i finally make an attempt are my chances increased with a lowered ph? Carol, you mentioned "Discus need soft water to hatch eggs." The natural water hardness here in New Zealand is 0-1dh, is this too soft?

One other question, you mention daily water changes and of a high percentage....how do you refill all that water at the same temperature? do you just use warm out of the tap of add some boiling water? do you fill buckets or use a hose? just curious.

thanks for all your help, it's great

Luca.

[Carol_Roberts]

I have moderately hard water and a pH of 7.8 in the tank. My well water comes out of hte ground under pressure with a pH of 6.8. I can't do 50% water changes with my tap water as it causes too much pH swing.

I have plastic 50 gallon storage barrels that I fill with water. Each barrel has an aquarium heater and a pump. I use the pump to "age" (circulate, airate, agitate) my water so the pH will match that in the aquariums. I have the heaters set at 84 degrees. I use all my water up during water changes and then refill the barrels for the next day.

Some people with soft water (maybe you?) don't have to worry about pH swing. Their tap water and tank water are pretty much the same pH. Those lucky folks can adjust the temperature of their tap water (add dechlor product to the tank) and fill directly from the tap with either a bucket or a hose.

With your soft water you may need to add buffers or minerals to avoid a pH crash.

[ronrca]

I have a question pertaining to the nitrogen cycle.

I have heard this a couple of times but never really understood it. As the biological cycle consumes ammonia/nitrites producing nitrates, the ph will slowly drop. Why?

[Carol_Roberts]

That biological process consumes carbonates and produces acids . . . .or something like that, lol. Anyway the nitrification process uses some minerals. This is only a problem in soft water. For most of us, water changes add back enough minerals to keep the tank stable.

[xpotato]

Thanks for the information.

I have a question to ask

Currently I am doing fishless cycling for my tank.
I had nitrites off the chat, 10 nitrates, and no ammonia.
Ihave made 10% WC several times but the nitrites is still off the chat.

What should I do to get rid of the nitrites?

[Carol_Roberts]

90% water change. Your tank is showing nitrAte you are cycled.

[ronrca]

I agree with Carol! After you do the 90% water change, test the cycle by adding again 5-8ppm ammonia and seeing how long it takes to 'cycle'. It should not take longer than 2-3 days for ammonia and nitrites to reach 0 again. Now you know that you can fully stock your tank.

Why do I mention this? Well, from my own experiences I would not test the cycle and add fish. What would happen is that ammonia is fine but the nitrites start rising again. This is due to the multiplication rate difference between the two strains of bacteria responsible for ammonia and nitrite. Just a tip! ;)

[Luca]

Hey Carol -
you say your tank water is ph7.8 but your well water is 6.8, does it rise in your storage container??

my ph has been crashing and swinging all over the place due to its 'blankness'. the kh is 2 the dh is 0 ph is 7.8

as a result my fish have got some strange black disease (i'm guessing from the stress of ph swings). the temperature was also fluctuating until i got a large filter up and running (via aqua 750), before that i just had an aqua clear mini on the tank (70gallon).

i think the first thing i should do is get these minerals i need added to my substrate, what do i do?

distressed.

[Carol_Roberts]

The pH rises in my storage barrels as the CO2 is off-gassed by water agitation. I don't know very much about soft water like yours . . . . April has soft water. Why don't you start a new post in the water section. I think there is a receipe there for the minerals needed to stabilize your water.

[delmore]

Optimum pH for sustainable biological activitiy (microflora) in the aquarium is stated to lie between pH 6.5 and 8.0...

When I fishless cycle my pH jumps from 7.4 -> 8.0+
The ammonia must cause the spike in the pH. Anyone know why this is? I am cycling a 55g bb tank now. Nitrite - 0,nitrate -0, ammonia - about 5ppm, pH around 8.

[Carol_Roberts]

I think ammonia itself raises the pH.

[delmore]

I think ammonia itself raises the pH.
Right. From speaking to others that have fishless cycled, sometimes the pH rises, sometimes not. I assume it has to do with the particular minerals in the water that make it "hard" - calcium carbonate or whatever. If the pH rises too much, the cycling process slow, so it is not a bad idea to monitor it.

[aziyaeian]

I understand why should I test the amount of ammonia and nitrite. But I can't find that why should I test the nitrate. I used to change water regularly so am I ought to test nitrate?
Does testing nitrate or nitrite help me to find the amount of ammonia?

[ronrca]

Depends on what you term regular (daily, weekly?)! Stocking density, feeding (what type of food and how often) and % of wc will all affect the nitrate level. For discus, very low or no nitrates are best therefore testing on a regular bases should establish your wc schedule. Depending on the setup, some tanks only may only require 25% daily where as others may need 75% daily to maintain very low nitrate levels.


HTH

[A.J.]

At what level does the amount of nitrates become harmful to your fish? I just did a test on the water of my fish tank and the ph was at 6.4, ammonia was at 0, nitrite was at 0, and the nitrate level was at 5.0.

[Hans Kloss]

At what level does the amount of nitrates become harmful to your fish?

Depending on fish strain. The most nitrates sensitive fish I ever kept were dwarf suckers (otos) and elephant nose (Gnathonemus petersii). Prolonged nitrates concentration of 30 ppm is unacceptable for them and they die during several months. Discus are not very sensitive to nitrates but they need clean water, free of other pollutants.
Hans

[wbzorker]

Discus are not very sensitive to nitrates but they need clean water, free of other pollutants.
Hans


Hi Hans,
Would you mind clarifying "other pollutants"? I'm thinking that you might mean the heavy metals, phosphates and liquid bio waste. Am I right?

Thank you for your time,

[Hans Kloss]

Hi Hans,
I'm thinking that you might mean the heavy metals, phosphates and liquid bio waste.

I've never seen discus poisoned with heavy metals or phosphates. In fact this kind of poisoning is almost impossible until you use municipal water. But the real problem are "bio wastes". No, they are not very harmful, they only can feed bacteria. But big amount of bacteria in water is potentially dangerous for discus and can result with gills, skin, fins and eyes infections. Personally I treat nitrates only as an indicator of overall (including bacterial) water pollution.
Hans

[wbzorker]

Thanks, Hans.

[RockHound]

Nitrogen is found in aquaculture water in two basic forms, i.e., un-oxidized and oxidized nitrogen, as follows:

Un-oxidized forms of nitrogen include:
Ammonia (NH3)
Organic nitrogen (Org-N)
Nitrogen gas (N2)

Oxidized forms of nitrogen include:
Nitrite (NO2)
Nitrate (NO3)
Nitrous oxide (N2 O)

Nitrogen typically enters water as unoxidized nitrogen, in the form of organic nitrogen (Org-N) and ammonia. The combination of ammonia and Org-N concentrations is generally reported as total Kjeldahl nitrogen (TKN).

Although bacteria have various nutritional requirements, the main source of their food is in the form of organic and inorganic carbon.

In terms of carbon requirements, bacteria can be classified as:

Autotrophic bacteria (usually referred to as autotrophs) use inorganic carbon, such as carbon dioxide and bicarbonate, as their food source (for growth and reproduction), but also require other inorganic chemicals, such as ammonia, to supply energy. Autotrophs are instrumental to the ammonia removal process.

Heterotrophic bacteria (usually referred to as heterotrophs) use the organic carbon in wastewater as their food source (for growth and reproduction) and as a source of energy. Heterotrophs are instrumental in removing organic material from water and are instrumental in completing the nitrogen removal process.

An important distinction between heterotrophs and autotrophs is that autotrophic bacteria expend more energy for growth than heterotrophs, resulting in lower growth rates among the autotrophs.

Note: both are vital to the total nitrogen removal process.

Bacteria are also grouped according to their need for molecular oxygen (dissolved oxygen = DO) as follows:

Type of Bacteria Oxygen Requirements

Aerobic Can only exist when there is a sufficient supply of dissolved oxygen (DO).

Anaerobic Can only exist in an environment with minimal DO and derive oxygen chemically for respiration.

Facultative Can exist under aerobic or anaerobic conditions; prefer aerobic environments, so if DO is present in the water, they will consume it first, then derive oxygen chemically by utilizing nitrate, sulfate and carbonate (in that specific order).

With respect to the types of bacteria listed above, facultative heterotrophs play the dominant role in removing dissolved organic pollutants during a biological water treatment process.

The rate of growth of each type of bacteria depends largely on temperature, the type of food and nutrients available and the availability of DO. As you might guess, given an environment that has an ample food supply, but limited oxygen, it is the facultative bacteria that possess the greatest potential for survival and growth!

Role of Nitrifying Bacteria in Nitrogen Conversion:

Before examining the role of nitrifying bacteria in nitrogen conversion, it is important to understand that before nitrification can occur, most of the organic material present in the water must be removed. The removal of this organic matter is typically measured in terms of Biochemical Oxygen Demand (BOD) and is accomplished during secondary treatment by providing an aerobic environment for aerobic and facultative heterotrophs to consume the organic carbon.

This stage of BOD removal is often referred to as first-stage BOD removal or cBOD removal.

Once the cBOD concentration is sufficiently lowered, conditions become optimal for the growth of nitrifying bacteria (provided that sufficient levels of DO and alkalinity are available). The nitrifying bacteria use the DO and inorganic carbon to convert ammonia to nitrite and nitrate. The oxidation of ammonia to nitrite and nitrate is sometimes referred to as nBOD removal or second-stage BOD removal.

With that said, let’s now examine the ammonia conversion process.

In the process of nitrification, ammonia nitrogen is converted to nitrate utilizing purely aerobic autotrophs - the Nitrosospira & Nitrospira group of bacteria. All of which bacteria are instrumental in completing nitrification, the nitrification reaction in a two-step process as follows:

Step 1:
Nitrosospira bacteria, which are purely aerobic autotrophs, convert ammonia nitrogen to nitrite. During the conversion process these bacteria consume large quantities of DO and reduce alkalinity.

NH3 + Nitrosospira Bacteria + O2 + Alkalinity NO2 (Nitrite)

Step 2:
Nitrospira bacteria, which are also purely aerobic autotrophs, convert the nitrite (produced by the
Nitrosospira bacteria) to nitrate. Once again, the vital element needed for this conversion to occur is DO and the conversion process further reduces alkalinity. Nitrosospira have a faster growth rate than Nitrospira, therefore, once the ammonia is converted to nitrite, the conversion to nitrate occurs rapidly.

NO2 + Nitrobacter Bacteria + O2 + Alkalinity NO3 (Nitrate)

Role of Denitrifying Bacteria in Nitrogen Removal

Now that you have a basic understanding of how nitrifying bacteria converts ammonia to nitrate during biological treatment, let’s examine the role of denitrifiers in nitrogen removal.

In the process of denitrification, nitrate, the form of nitrogen that results from the completion of the nitrification process, is converted to nitrogen gas, utilizing facultative heterotrophic bacteria. The process of denitrification occurs under anoxic conditions as follows:

NO3 + Denitrifying Bacteria + Organic Carbon Nitrogen Gas + Water + Alkalinity

There are two steps required to achieve denitrification, i.e. nitrate removal.

Step 1- The nitrification process is reversed, and nitrate (NO3) is converted back to nitrite (NO2).

Step 2- Nitrite is converted to nitric oxide (NO), then nitrous oxide (N2O) and finally to nitrogen gas (N2).

The completion of the denitrification process can be summarized as follows:

NO3 > NO2 > NO > N2O > N2

There are two main requirements for successful denitrification.

1. Anoxic environment
Since facultative heterotrophs prefer to respire using DO, the denitrifiers will continue to respire
aerobically as long as DO is available. It is only when the DO is depleted that denitrifiers begin using nitrate for respiration, which begins the denitrification reaction.

2. Sufficient amount of organic carbon
Without organic carbon as the food supply, facultative heterotrophs cannot continue to grow,
multiply and thrive. Organic carbon can be supplied to the denitrification phase of biological treatment using influent water (which contains cBOD), and/or supplemented with either an ethanol, or a sugar solution.

One final point: Biological denitrification converts nitrate to nitrogen gas. Which is typically gassed off in an aerated chamber that follows the denitrification process.

[Hans Kloss]

Excellent summary, RockHound. Must-read for every fishkeeper.
Hans

[illusionkid6]

Just the article I needed to read. Working on setting up my first Discus tank and need all the help I can get.

[Apistomaster]

Plenty of good info in this thread but I want to address the myth that biological filtration does not work in extremely soft and acid water. For example a tank with 20 ppm TDS and a pH between 3.5 and 5.0.
There are species of bacteria that do fill this niche. I think the myth that bio-filtration doesn't work in the lower range came from experiences where hobbyists made change too quickly.

I have run tanks with a TDs of only 20 to 25 ppm TDS at a (pH between 3.5 and 4.0 for several years at a time.)
In this more extreme soft and acid water the bacteria which are adapted to the low end do the job but are not as efficient as those that prosper at more typical ranges.
Just kept it in mind and allow the Discus more space than the commonly recommended 10 gal per adult Discus. The system will work and Heckels and Greens will thrive.

There is no justification for keeping any Domestic Hybrids or wild S. haraldi in such extreme low hardness and pH parameters as they are naturally adapted to thrive in water which is considered by most to be "normal".

[Kevinh]

Really Good information. My tank is new (3.5 weeks) and this explains why my nitrites level is raising. So I'll start doing more water changes and monitor. Thanks

[cro117]

awsome post rockhound. i was going to add my simple explanation of denitrification, but you did it way better. i'm guessing you work in water treatment? i guess the only thing i have to add to this thread then is a bit more of an explanation of why nitrification lowers the ph, most of the time not even noticable.

heres a simplification of nitrification:

organic + 175 O2 → 122 CO2 + 16 NO3- + 16 H+ + 138 H2O ------- so basicly oxygen+waste > nitrate + carbon dioxide +water + H+ ion

the significance is that ph is simply just ions. water is H20 and if you split those you get H+ and OH-. H+ is acidic, OH- is alkaline. if you have very soft water and low carbonates and bicarbonates the ph swing will be a little more significant. so if you have well stocked tank turning over a significant amount of organics all the time, you will notice the ph getting lower every year.

[cro117]

oh, and as to rockhounds post, you can achieve a completed nitrogen cycle in your own tank extremely simple by adding a deep sand bed. just be careful as there are gases dangerous to your fish produced in the lower levels. if left undisturbed they are neutralized as they diffuse up to the surface and are oxygenated. so as long as you don't say pick up a deeply embedded rock disturbing the sand or something they are nothing to worry about. every tank i have i use a deep sand bed, and i have never had a sulphate die-off.

as far as a carbon source, detritus that makes its way below the sand will act as a descent source, but you can enhance the process by adding others.