How to Lower Nitrites in Your Fish Tank: 11 Step Guide

2. What Are Nitrites?

Nitrites are dissolved nitrogen (N) that occurs naturally in your fish aquarium. They’re a part of the nitrogen cycle (something all beginners should familiarize themselves with!) and typically, are eventually converted into nitrates.

Nitrites come from the breakdown of organic materials in the tank. All sorts of organic matter can increase nitrites: Waste (poop) from aquatic life, decaying or dead plants, leftover fish food, a dead fish or invertebrate, overcrowding, poor water quality maintenance – All these things can do it. They start to decompose, convert into ammonia, and then end up as nitrate.

When tank inhabitants eat and then generate biowaste and poop, it creates toxic compounds like ammonia. That good bacteria I mentioned earlier that occur naturally in an established tank “eats” the ammonia, helping to purify the tank water. Balanced water chemistry keeps your fish healthy and alive.

The presence of nitrite in a tank is an indicator that the tank’s nitrogen cycle is disrupted: This could mean the nitrogen cycle hasn’t fully finished developing in a new tank, or that something has gone awry in an established tank (overstocking, introduction of a new chemical, etc.) We’ll explain this fully in a moment.

PS- Understanding the nitrogen cycle is CRUCIAL for fishkeeping. But if you’re already knowledgeable about the nitrogen cycle and want to skip to the part about how to lower nitrites, you can click here.

3. Where Does Nitrite Come From In The Fish Tank?

Nitrite is a byproduct of the breakdown and decomposition of organic materials in the tank (like decaying plants or fish waste). Biowaste, decaying or dead plant remains, dirty filters, leftover food, and overstocking the tank all can result in nitrite or increased nitrite levels in your tank. If you already know about how ammonia spikes happen, then this will sound familiar. Testing aquarium water for ammonia, nitrites, and nitrates is critical. It’s also worth testing your tap water in case it already contains some of these (unfortunately, it can happen).

4. What Is A Nitrogen Cycle?

Remember how I said that the presence of nitrites is an indicator that the nitrogen cycle is disrupted? Here’s what this means, and why it matters:

The nitrogen cycle is the cycle in which ammonia is converted into nitrogen-containing compounds (nitrite and nitrates). This cycle is super important because it makes the tank inhabitable for aquatic life. Otherwise, your fish would continue pooping and generating biowaste, and it would immediately become ammonia, killing the fish.

This is why when you set up a new tank, you have to cycle it first until the chemistry is balanced (eg, the ammonia conversion process), therefore making the water safe for fish. This is also why so many beginners find their fish die when they didn’t know to cycle the tank first, or when they did drastic water changes (which removes too much of the good bacteria, causing an ammonia spike).

During the nitrogen cycle, ammonia (produced from poop and other biowaste) is converted into nitrite by good bacteria (nitrosomonas). Next, this nitrite is converted into nitrate by another good bacteria (nitrospira).

While the nitrogen cycle is becoming established (eg, cycling a new tank), ammonia will first go up, leading to detectable levels of nitrite. As the nitrite becomes detectable, the ammonia will inversely start going down.

In the next stage of the nitrogen cycle, there will be a fall in the level of nitrite while levels of nitrate will increase.

In a cycled tank, there’s enough good bacteria that the nitrite is converted immediately to nitrate – So your nitrite levels should read at 0.

The presence of nitrate in the tank means the tank is experiencing the full nitrogen cycle.

a. Difference Between Nitrate And Nitrite 

This one can be easy to confuse. Nitrite and nitrate are two different compounds produced from ammonia (which comes from poop, biowaste, decaying organic matter, etc.) during the nitrogen cycle. They are converted from ammonia with the help of beneficial, nitrying bacteria. It goes: Biowaste/poop/etc. >> Ammonia >> Nitrite >> Nitrate.

These two compounds are different, and their level of toxicity is different too.

Both compounds (NO2 and NO3) are toxic to aquarium fish and invertebrates. However, their toxic concentration is different. Nitrites above 0.5 are dangerous for the inhabitants of the tank, while fish can survive at nitrate levels from 0 to 20 ppm. Technically, they can even survive up to 40ppm nitrates, but at those levels, there’s significant risk to the fish. (That’s why you’ll see online the range of 0 – 40ppm listed).

This is why it’s important to know BOTH of these compounds, how to test for them, and how to lower them.

The summary is: When ammonia is generated in the tank, good, nitrifying bacteria present in the tank helps convert it to nitrites. In the later stage, nitrites (NO2) are converted into nitrates (NO3). At that point, your plants, water changes, etc help control the nitrate levels to keep the water clean for your fish.

5. How Do You Test For Nitrites?

Checking the nitrite levels is easy. You use a colorimetric test kit designed specifically for nitrite testing. Nitrite test kits are easily available online or at your local aquarium store. I definitely recommend stocking up as it’s not something you want to run out of!

The process of testing nitrite levels in both saltwater tanks and freshwater aquariums is pretty much the same as the testing procedure for pH. If you’ve ever done a pH test, then you’ll know how nitrate and nitrite test kits work. The color of the test strip changes according to the water chemistry, and you use a key to decipher it – Just like in science class!

The process is the same for both saltwater and freshwater. Here’s how it works:

• First, take a small water sample from your fish aquarium using a clean container.
• Add a few drops of the solution that came with the kit; The nitrite test kit will have instructions on exactly how to do this.
• Wait and watch the color change.
• Match the test strip to the color key provided with the kit.

6. What Is An Acceptable Nitrite Level In A Freshwater Fish Tank?

The optimal nitrite level is 0. Yes, zero. If the nitrite goes above zero, the fish in the tank will encounter stress. Nitrite levels greater than 5 ppm (parts per million) can be extremely toxic for fish.

There has to be enough good bacteria in a tank to break down nitrites immediately. If your nitrite levels are above 0, you’ll want to immediately work to determine the cause and lower the nitrites (which we’ll get to in section 9).

7. What Happens If Your Aquarium Nitrite Levels Are High?

a. Nitrite Poisoning

Nitrite poisoning occurs because of high nitrites in aquarium water. A sudden nitrite spike may occur because of excess waste and remains of dead fish, for example. The raised level of nitrite in the tank may result (in severe cases) in nitrite poisoning. A sudden nitrite increase can result in the death of your fish, similar to a dreaded ammonia spike.

Nitrite poisoning is also called “brown blood disease,” on account of the fact that it turns the fish’s blood brown. Awful, right?

This is because nitrite causes an increase of methemoglobin, which is responsible for changing the color of the blood and causing several other harms. It disables the blood’s ability to carry oxygen, and your fish suffocates.

Fish die in water with elevated ammonia and high nitrite. This is why it’s absolutely crucial to eliminate them, and to know how they work.

b. Increased Stress

Nitrite in the fish tank increases stress to the fish, causing them to become vulnerable to disease and illness, and even a shortened lifespan.

c. Underdeveloped Growth

Nitrites can also affect the growth of your fish, causing underdevelopment and stunted growth.

d. Suffocation

Nitrite stops the transport of oxygen. This makes breathing hard for the inhabitants of the tank. You may find your fish frequently going to the water’s surface to breathe; Fish will die because of a lack of oxygen in the blood.

e. Poor Coloration

With nitrites in the tank, the coloration of your fish will also be affected, due to the increase of methemoglobin. You may notice dull coloration.

8. Signs Of Nitrite Poisoning In Aquarium

With nitrite poisoning, your fish will be under stress and may show several signs that indicate the water chemistry is unbalanced. It’s important to note that some fish are hardier than others and may not show negative effects until it’s too late, so water testing is crucial.

Here are some possible signs of nitrite poisoning:

• Change in swimming manner (including side or upside-down swimming)
• Curved spine/body; The fish looks curled up
• Eccentric movements
• Lethargic behavior
• Lack of energy and activity
• Rejecting food
• Shortness of breath
• Pale, dull, or changed body color

9. How to Protect Your Fish During A Nitrite Emergency

You should test for nitrites weekly. If you find nitrite in your tank, immediately work to delimit its toxic impact on the aquatic life in the tank:

• Transfer Fish To an Established Tank

To protect tank inhabitants, transfer them to an already-established tank. This is the most effective first measure to tackle the nitrite emergency as it removes the immediate, life-threatening risk to your creatures. This is one of those times that it pays to have good relationships with other local aquarists, or even a local aquarium store.

If you can’t access an established tank, there are other steps you can take:

• Add Rock Salt

You can add rock salt to the tank. Rock salt will increase chloride levels in the water. Increased chloride levels help regulate excess nitrites. In addition, rock salt can help deflect the risk of harmful excessive nitrates entering the fish’s body, helping to reduce toxicity. An alternative here is to use 1 teaspoon of non-iodized salt. The recommended amount of non-iodized salt is 1 tablespoon per every 10 gallons.

•  Add An Air-Stone

During nitrite toxicity, fish in the tank may encounter difficulty breathing. Nitrites enter the bloodstream, react with hemoglobin, and result in methemoglobin. Methemoglobin lacks oxygen and this leads to suffocation and difficulty in breathing. You can use air stones in the tank to support and increase the oxygen level, helping your fish breathe.

Then, you’ll want to immediately work on lowering nitrites in your fish tank.

10. How to Lower Nitrites In A Fish Tank in 11 Steps

a. Partial Water Change

As soon as you detect nitrite in your tank, immediately perform a partial water change. Depending on the severity of the nitrite detected, you can change 25-50%.

Check the nitrate and nitrite level of your tap water before using it for the water change, just to be on the safe side. You can use a water conditioner to help get rid of chloramine in the tap water if needed.

Doing a water change dilutes the content of nitrite present in the fish aquarium. This will lead to the better functioning of nitrifying bacteria and the nitrogen cycle (in freshwater and saltwater fish tank).

Please note that it’s possible to overdo it here: Excessive water changes can have a negative impact on aquarium ecology and aquatic life, because while it does remove negative chemicals, it also removes good bacteria that you need for the nitrogen cycle. Removing too much water too frequently can remove too much good bacteria, causing an ammonia spike.

Work on cleaning up your tank; Use a gravel vacuum to effectively remove the biowaste of fish, leftover or uneaten food, and other sources of ammonia present in the tank.

b. Nitrite Removing Liquid (Chemical Detoxifying Agent)

Nitrate levels of the tank can be removed by adding specially designed detoxifying agents. Add the amount of the liquid provided in the product instructions. In an established tank, you’ll see a nitrate curve reduction over time.

This should really only be used in a serious nitrite spike situation, when you can’t move the tank inhabitants. It’s still crucial to determine the cause so you that you can fully fix the issue. It’s not a substitute for a healthy nitrogen cycle.

c. Add Aquarium Salt

Adding aquarium salt to the tank can also reduce the increased levels of nitrite in case of a nitrite spike. (As you probably guessed, this doesn’t really work in saltwater tanks). You can get aquarium-safe salt from any local fish store. The usual recommendation is somewhere around 1 tablespoon per every 5 gallons in a freshwater aquarium, but follow the ratios on the product’s directions.

This amount of salt shouldn’t cause stress to aquatic life, even for salt-sensitive fish like tetras and betta fish. That said, you should research the species of fish in your tank before adding it to make sure that there aren’t any particularly intolerant (there are several freshwater fish species that find it too stressful).

Aquarium salt is pretty useful as a medication for lowering nitrite in a freshwater tank. I recommend keeping a bottle at home for emergencies. (It can also come in handy for other things, like treating parasites.)

d. Add Beneficial Bottled Bacteria

You can help lower nitrite in a fish aquarium by using beneficial bottled bacteria. It’s better to add nitrifying bottled bacteria after a partial water change.

Nitrifying bottled bacteria is always helpful to have on hand, as it regulates the nitrogen cycle quickly.

e. Add Water Conditioner

Adding water conditioner in the tank in case of high nitrite levels is an instant method. Water conditioners are used to remove nitrite content from unbalanced water; They’re easy to use and effective. You can pick it up at your local aquarium store, and add it to the water according to the label instructions. Don’t overdo it. Follow the steps on the bottle and watch how it lowers the nitrite spike.

Water conditioners work by attaching to nitrite, detoxifying the tank water and regulating the chemistry of the water.

These conditioners or “instant nitrite removers” only reduce the level of the nitrite; They don’t resolve the cause of the increased nitrites. Meaning, if you don’t solve the problem and fix the tank’s nitrogen cycle, the nitrite spike will just come right back.

Water conditioners don’t affect the aquatic life present in the tank. They also give filter bacteria the chance to effectively convert nitrites into nitrates.

f. Reduce Livestock

There is a direct relationship between increased nitrite levels to the amount of ammonia being produced by the aquatic life of the tank (in the form of biowaste).

If you have a large number of fish in your tank, they’ll create a large amount of biowaste. Don’t overstock; It’ll have a directly negative impact on nitrites and nitrates.

If you’re stocking a new tank, you should add one species at a time, then test the water over time before adding more, so as to ensure the tank can handle it.

Overfeeding should also be avoided, as the leftovers become ammonia (and therefore, later, nitrites and nitrates).

11. What Are The Causes Of High Nitrite Levels In Tanks?

Nitrites are the byproduct formed when nitrifying bacteria convert ammonia as a type of biological filter for their compounds. The build-up of an excessive amount of these nutrients can be caused by overfeeding, overcrowding, or other poor maintenance and disturbed nitrogen cycle.

• Overfeeding

One cause of a rise of nitrite levels is overfeeding. It can lead to the buildup of excessive nutrients in the water column; As a result, algae grows in the tank and has a negative impact on the quality of water.

The uneaten food or remains of leftover fish feed decomposes, releasing ammonia, which is toxic to fish and invertebrates. The nitrogen cycle then leads to the formation of an excessive amount of nitrites.

• Overcrowding

Overcrowding is another culprit for increased nitrite levels in fish tanks. Overcrowding a tank leads to the production of large amounts of biowaste. This increased amount of waste results in extra ammonia. In the later stage of the nitrogen cycle, this ammonia converts into nitrite and nitrates.

In sum: The more fish you have, the more waste they make, and therefore the more ammonia they make. Ammonia is converted into nitrites and then nitrates, so more ammonia = more nitrites/nitrates. This is why I recommend adding fish to a tank gradually instead of all at once, so that you can test in between and make sure the tank can handle it.

• Poor Maintenance

Fish keepers have to be diligent when it comes to maintenance. Regular, scheduled maintenance and testing is a big must. This lets you know exactly what’s going on in your tank, and allows you to catch any issues in a timely manner. Keenly monitoring and maintaining water chemistry, quality, and parameters is crucial for a healthy and thriving tank, and regular maintenance (such as partial water changes, siphoning substrate, cleaning, and testing) is the cornerstone of that. Check important water chemistry components like pH, nitrites, nitrates, ammonia, etc, weekly.

Note that dirty substrate or overfilled equipment (eg, filters) and leaving dead organic matter in the tank can both lead to the production of excessive amounts of nutrients (ammonia spike, nitrite, and nitrate) and dangerous nitrite levels in aquarium water.

But don’t take my word for it:

Skipping maintenance can lead to generous chemical spikes. Monitor both new and established tanks diligently, and keep live plants in your aquarium to help keep tank water oxygenated and mirror a natural environment for your fish.

• Defective Filter

This one is sneaky! Defective filters can also lead to an increase in nitrites or nitrates in the tank. Make sure your filter is powerful enough to keep up with the waste produced by the fish, and check your filter regularly both to clean it and to make sure you’ll catch it if it conks out.

12. How To Prevent Nitrites From Building Up?

Nitrite build ups happen. But by taking preventative measures, you can decrease the risk of nitrites in your tank. These are mostly things we’ve already gone over, but I’m summarizing them here – You know, in case you want to print it out, make a checklist, frame it above your fish tank… 😛

a. Test Your Water Regularly!

Use water testing kits to check your water chemistry. Pay particular attention to any ammonia levels in the water. A rise in levels of ammonia can indicate and/or foreshadow nitrite spikes. Plus, ammonia is dangerous for fish. Without a good water testing kit, you can’t manage your water chemistry. Using these regularly can save your fish’s life.

b. Don’t Overfeed

Learn about how the individual species you’re keeping feed; This can help your feed them more efficiently. Feed in small quantities, and remove any leftover food from the tank and substrate. Avoid feeding in large quantities and then leaving (for example, dumping a bunch of food in the tank and then leaving for work). You can also feed with the help of an automatic feeder, so you can feed in small quantities throughout the day as needed.

c. Water Change

Regular partial water changes are crucial. They help keep ammonia, nitrites, and nitrates under control. Opt for small, frequent water changes over large, infrequent ones, to help prevent removing too much good bacteria (and thus causing an ammonia spike).

13. How Long Can Fish Live With High Nitrites In The Fish Tank?

The answer to this question really depends on how high the nitrites are and how hardy the fish is, but with high ammonia and/or nitrites in the tank, aquarium fish might be able to survive for a few hours. This is because their blood will lack oxygen, making it impossible or very difficult for them to breathe.

If only the nitrite level is high in the tank, your fish might be able to make it for a few days, but it will be under extreme pain.

If you find redness around the gills of fish, heavy breathing, or fish gasping for air, then you need to immediately take action. Move the fish to an established tank, check the water chemistry of the tank, and take effective measures to balance it.

14. What Is The Fastest Way To Lower Nitrites In An Aquarium?

The quickest way to reduce nitrite levels in your tank is to conduct a partial water change (20 to 50 % water). This will dilute ppm and lower the toxic nitrite. In the case of a spike in toxic compounds, I recommend doing this immediately.

15. What Are The Steps In Which Ammonia Converts To Nitrates?

Ammonia present naturally in the tank converts into nitrites and nitrate in a series of steps (4 in total). Note that these steps only occur in a healthy, fully-cycled tank:

• Step 1

In the first step, biowaste and fish waste produced by fish, decaying or dead plants or animals, and remains of food breaks down (decomposition). During decomposition, they release ammonia.

• Step 2

This naturally-produced ammonia is poisonous to aquatic life in your tank. It is converted into nitrite by good bacteria.

• Step 3

This by-product (nitrite) is also destructive to the aquatic life of your tank. Fortunately, this nitrite is converted into nitrate by more beneficial bacteria.

• Step 4

In a cycled, established tank, the nitrate is immediately absorbed by plants or algae (it helps them grow). In this way, it’s not dangerous for fish because it’s immediately absorbed.

Ammonia, nitrites, and nitrates are all dangerous and can be lethal to fish and invertebrates. This is why it’s so important to understand the nitrogen cycle, and to continually ensure that your tank is cycled correctly. This is also why maintenance and partial water changes are important; Because they help reduce these toxic compounds.

16. How To Lower Nitrites In A Fish Tank: FAQs

a. How Do You Fix High Nitrites In Aquarium Water?

You’ll need a test kit that tells you the exact level of nitrite content present in the tank. Move fish to an established tank, to prevent their injury or death. Then, you have to immediately take effective measures to balance the water chemistry of the tank. Perform a partial water change (25-50%, depending on how dire your nitrite spike is) and continue this every 24 hrs until your nitrite reading is at 0.

This will help you eliminate remains or traces of ammonia and lower the nitrite level.

You should also use a vacuum to remove detritus and uneaten food from the substrate, but don’t overdo it as it too can lead to fluctuations in water parameters and disturbance of the nitrogen cycle.

b. How Can I Lower My Nitrate Levels Naturally?

This depends on how bad your nitrate levels are; A partial water change is the first step in battling high nitrites or nitrates, after moving your fish. It’s also a natural method. But if you have a high level of nitrates or nitrites, you need to reduce them immediately or your fish will die. For this, you will need a water conditioner.

In the long-term, or for lower levels of nitrates, adding more plants in the tank can reduce nitrite and nitrate levels in the tank. This is because plants will absorb these nutrients for their growth.

The key to controlling the nitrate level is to keep your tank clean and your water quality excellent.

c. Can High Nitrites Kill Fish?

Yes, definitely. High levels of nitrites in a tank will kill your fish and invertebrates. The chemical structure of nitrite means it’ll bond with blood when it enters the fish’s bloodstream. Under chemical reaction, blood is then deprived of oxygen and fish will face breathing issues and suffocation.

Poor water quality in the tank tends to create nitrite poisoning, in which aquatic life suffocates due to a lack of oxygen in the bloodstream.

Nitrite is more harmful than nitrate. It can affect the growth and reproduction of fish, and extreme excess nitrate may result in death.

d. High Nitrite Readings But 0 Ammonia?

This basically just means your nitrogen cycle is only halfway done. If you have high nitrite readings but 0 ammonia, this is because the ammonia previously present in the tank has converted into nitrite. This indicates the lack of the beneficial bacteria that are responsible for breaking nitrites down into other compounds (nitrates).

If your tank is new, you can’t add fish on day 1 because the nitrogen cycle isn’t complete. Oftentimes new aquarists will find their fish dying in their new tank, with no clue why. This is the most common reason (eg, not cycling a tank beforehand).

e. What Is An Acceptable Nitrite And Nitrite Level In a Freshwater Fish Tank?

If nitrite levels reach above 0.75 ppm in the tank, your fish and invertebrates will be under stress. Further increase in the nitrite levels (more than 5 ppm) can cause serious breathing issues in fish and eventual death.

Nitrate levels from 0 – 20 ppm (parts per million) are safe for fish. 40 ppm is a dangerous level; it should stay below 20 ppm. If it increases above 40 ppm, it becomes detrimental to their health.

f. Do I Need a Biological Filter For My Fish Tank? 

When it comes to the water chemistry of your fish tank, maintaining the growth of bacteria (beneficial bacteria) is vital to change ammonia into nitrite. Biological filtration and the conversion of ammonia and nitrite happens naturally in most fish tanks that are a few months old. New aquarium filters available on the market come with a special area that provides optimal conditions for the effective growth of good bacteria.

Ordinary filters are less effective in this way, and the bacteria will stay in the filter and decorations in the fish tank (on the substrate, rocks, and other decoration items). The newer filter styles that are designed to harbor beneficial bacteria can help keep ammonia and nitrites in check.

If your fish tank is stocked to capacity, and/or you have fish with a high bioload, these filters can be a good supplementary option.

g. What Causes High Nitrate Levels In Aquariums?

The key causes of high nitrate levels in a fish tank are decaying organic matter (from overstocking, dead leaves or fish, excess food, etc) and/or a disruption of the nitrogen cycle (such as accidentally introducing a harmful chemical to the fish tank).

Too many fish in a tank (overstocking) will produce a large amount of biowaste, affecting the natural nitrogen cycle. This will damage the biological filter you created when establishing and cycling your new, and lead to the formation of ammonia and nitrite.

There are a few sneaky causes, though, like failure of filter, delay in changing tank water, and not removing the decaying organic matter (dead leaves, dead plants, fish, algae, remains of food) from the tank.

h. Effective Plants To Remove Nitrates From Fish Tanks

Plants love to use nitrate as this helps them to grow effectively. In return, nitrate gets removed from the tank. I recommend you add plants to your tank, as it’s beneficial and effective. Some great options are:

• Moneywort
• Water wisteria
• Duckweed
• Amazon Frogbit
• Java Ferns
• Tall Hairgrass

For more details, you can check out this article.

i. What Are Nitrates? Recap

Nitrates are formed in the tank because of a disrupted nitrogen cycle, caused by things like overfeeding, overcrowding, or poor maintenance of the fish tank.

A good nitrate level for a freshwater fish tank is 0.5 to 5.0 ppm. In this range, your fish will thrive and it also encourages the healthy growth of live plants while keeping algae at bay. Too high levels of nitrates can cause harm to fish, invertebrates, and live plants.

18. Conclusion 

Nitrite is formed as a result of the nitrogen cycle that’s always running in your fish tank. It’s REALLY important to understand this cycle, and to know how to lower nitrites.

These compounds come for decaying organic matter and/or a disruption in the nitrogen cycle. Overstocking, not cycling a new tank, drastic water changes, infrequent water changes, poor tank maintenance, and overfeeding can all be culprits.

Sometimes, even tap water can contain nitrites.

If the level of nitrite increases above 0, fish will be under stress and their ability to breathe, grow, and reproduce will be negatively affected.

You’ll only be able to manage your water chemistry if you have a test kit and use it regularly. Clean your tank on a regular schedule, do weekly partial water changes, and test diligently. Remove uneaten food right away, and use a feeder dispenser if needed so you can feed smaller amounts. Observe your fish for any changes in behavior, and when needed, use the methods we discussed to lower nitrites in your fish tank.

Additional Sources/Resources/Reading

https://aquaponicsadvisor.com/the-best-way-to-remove-nitrate-from-a-fish-tank/

https://www.aquariumcoop.com/blogs/aquarium/nitrate

https://badmanstropicalfish.com/high-nitrite-in-aquarium/

https://www.aquariadise.com/high-nitrites-in-the-aquarium/

http://www.aquascapinglab.com/en/2017/07/06/nitriti-e-nitrati-in-acquario-cosa-sono-i-valori-no2-e-no3/

https://www.aquaticexperts.com/blogs/blog/nitrogen-cycle-freshwater