Aquarium Nitrogen Cycle - New Tank Syndrome

Updated May 13, 2020
Author: Mike - FishLore Admin
Social Media: FishLore on Social Media

The aquarium nitrogen cycle information presented below may be rather boring to most people, but it is absolutely essential to understand this process if you want to be successful at keeping fish!

Table of Contents

Aquarium Nitrogen Cycle Overview

Steps in the Process:

  1. Fish Waste & other biological processes

  2. Ammonia

  3. Nitrites

  4. Nitrates

  5. Water changes to remove nitrates and DOC

Aquarium Nitrogen Cycle
Photo Credit: Ilmari Karonen

Some call it the biological cycle, the nitrification process, new tank syndrome or even the start-up cycle. They all are referring to the same cycle - the aquarium nitrogen cycle. The cycle is a very important process for the establishment of beneficial bacteria in the aquarium and in the filter media that will help in the conversion of ammonia to nitrite and then the conversion of nitrite to nitrates.

How long does this process take? Well, timeline wise this process can take from 2 weeks to 2 months or longer to complete. It is vital for anyone planning on keeping aquarium fish to learn and understand this process!

The best way to monitor the cycle is to purchase an aquarium test kit that will test for ammonia, nitrites, nitrates and pH. While you certainly can get the individiual test kits for ammonia, nitrite, nitrate and pH... the cheapest option is to get a master test kit that contains all of these tests in one kit.

Test your aquarium water every other day and write down your test readings. You will first see ammonia levels rising. A few weeks or so later you should see the nitrite levels rising and the ammonia levels dropping. Finally, after a few more weeks you should see the nitrate levels rising and the nitrite levels dropping. When you no longer detect ammonia or nitrites but you can detect nitrates you can assume that it is safe to add fish to your aquarium.


Aquarium Nitrogen Cycle Stages

Stage 1
Ammonia is introduced into the aquarium via tropical fish waste and uneaten food. The tropical fish waste and excess food will break down into either ionized ammonium (NH4) or un-ionized ammonia (NH3). Ammonium is not harmful to tropical fish but ammonia is. Ammonia is very toxic to fish. Whether the material turns into ammonium or ammonia depends on the ph level of the water. If the ph is under 7, you will have ammonium. If the ph is 7 or higher you will have ammonia.

Stage 2
Soon, bacteria called nitrosomonas will develop and they will oxidize the ammonia in the tank, essentially eliminating it. The byproduct of ammonia oxidation is Nitrites. So we no longer have ammonia in the tank, but we now have another toxin to deal with - Nitrites. Nitrites are just as toxic to fish as ammonia. If you have a test kit, you should be able to see the nitrite levels rise around the end of the first or second week.

Stage 3
Bacteria called nitrobacter will develop and they will convert the nitrites into nitrates. Nitrates are not as harmful to tropical fish as ammonia or nitrites, but nitrate is still harmful in large amounts. The quickest way to rid your aquarium of nitrates is to perform partial Water changes. Once your tank is established you will need to monitor your tank water for high nitrate levels and perform partial water changes as necessary.

There are other methods to control nitrates in aquariums besides water changes. For freshwater fish tanks, live aquarium plants will use up some of the nitrates. There are now products you can run in your filter to remove nitrates. One such product is made by Seachem and is called Denitrate. For saltwater fish tanks, live rock and deep sand beds can have anaerobic areas where denitrifying bacteria can breakdown nitrates into harmless nitrogen gas that escapes through the water surface of the aquarium. While nitrate is not as harmful as ammonia and nitrite, you still don't won't it getting too high and you need to figure out your plan for keeping it low for long term success (e.g. water changes, live plants, live rock, filter supplements, etc).

Starting The Cycle With Fish

This is not the preferred way to get the cycle started because the fish are being exposed to ammonia and nitrites during this process. Many fish can not and will not make it through the cycling process. Often times the fish become stressed and fish disease starts to break out. I wonder what percentage of disease is caused by the cycling of new aquariums?

Certain fish species are hardier than others and seem to tolerate the start-up cycle better than others. For freshwater tanks, the zebra danio is a very hardy fish that many use to get the nitrogen cycle started. For saltwater tanks, some have reported success using damselfish to get the process started. Again, using fish to cycle is not a good idea and you may be throwing your money (on dead fish) out the window. But... if you do have fish in a new tank and you need help, be ready for frequent testing, frequent water changes and the self loathing after the realization that you are putting your fish through misery when it could have totally been avoided. Read the following on how to get through a fish in cycle: Cycling With Fish.

There is an easier and better way. Read on, young grasshopper.

Starting The Cycle Without Fish

There are a few different ways to get this process started. To easily get an ammonia reading from your tank water try the Seachem Ammonia Alert. It sticks inside the tank and has a circle that changes color depending on the ammonia levels in the tank. It doesn't seem to have the most accurate measurement so you would need to get a good liquid ammonia test kit to get a more accurate reading of the ammonia levels in your tank.

Once the cycle has started only add one or two fish at a time. Wait a couple of weeks before adding more fish. This will give your tank the time it needs to catch up with the increased bio-load.

Speeding Up the Cycling Process

There are things you can do to speed along the process of cycling your aquarium.

Aquarium Cycle Comments

From: Dave M.
To start the CYCLE, one should never use fish food! Most, if not all, fish food (flake or otherwise) contain heavy amounts of phosphate. Once the Nitrogen Cycle is in it's final stages, the first thing you will notice is an explosion of all types of algae. Phosphates encourage it's growth! Don't tamper with the Ph, till after the Cycle is complete. Afterwards, try to keep the Ph low, around 7.0-7.2, as a high alkaline also increases algae growth. Purchase several (cheap) "Bunched Plants" and place them in the tank. No need to draw them to the bottom; just throw them in, if you like, as you probably won't need or want them anymore after your landscaping plants and the aquarium establishes itself. This method will "out-compete" the algae and your "Planted Tank" will be virtually free of this pesky micro-plant.
Thanks for allowing my input on the Aquarium Cycle.

From: Brandon J.
Never rinse your filter media with regular tap water if it becomes clogged. The chlorine in the tap water will kill your benificial bacteria instantly and you will have to start all over. Then you will see an explosion of ammonia and nitrites. Rinse your filter media in tank water if you must rinse due to over feeding. Always feed no more than yor fish can eat in 2-3 minutes to avoid filter media clogging which can cause over spill and less water going through your bacteria media culture. Good luck!

From: Mike
SeaChem also came out with a product called Stability, which works great to speed up the cycle. I've used Cycle and Stability and I thought Stability worked a little better. I've heard Marineland's Bio Spira is great too but it's hard to find. Marineland's head scientist is an expert in Beneficial Bacteria... so I trust their stuff a lot.

From: Cody
I used the Bio-Spira in my tank only a couple of days after setting it up from scratch and it worked absolutely fantastically. I have not since ever seen a spike in ammonia, nitrites, or nitrates. My tank is now two months old and I was able to add fish in the first few days after setting it up. I highly recommend this product to any new setups.

From: Carsten
In Feb 2007 my brother passed away and his wife could not handle dealing with an aquarium. So, I brought them home 700 miles in a 2 gallon cooler also with his filter, media and sorted stuff. After I got home I needed to setup an Aquarium quickly. I had one when I was younger and know a little. I left them in the cooler with a heater, plants and air stones for 3 days. I monitored the water chemistry since I wanted my brothers fish to live. I saw the ammonia levels rising, etc. So when I visited my local fish shop they recommend I used the bio-spira. Thank God for that stuff, my tank started to cycle quickly, ammonia was there then after a day with Bio-Spira nitrites, then Nitrates kicked in, and ammonia's started down. Worked like a charm and they are all well.

One other experience, I overcleaned my tank, back to the LFS, more Bio-Spira to fix the issue. I've also learned that I never clean the tank and change out the filters at the same time. Another great product I keep on hand is Prime. Dosing with Prime will help get ammonia levels down while the cycle is stabilizing. Setting up a new 55 gallon today and ran across this great site. Thanks for all the efforts.

From: HSimpson - about Septo-Bac
Septo-Bac can be used to speed up tank cycling and it is a fraction of the cost (a box of 12 packages of Septo-Bac costs about $5 and can be used to cycle 10 150 gallon tanks) of more expensive products such as cycle, bio-spira, stability, and stresszyme.

I did not post this to market Septo-Bac and for those who doubt me, you can do a google search about other peoples' experience to use Septo-bac to quickly cycle large tanks. I can honestly tell you that I was able to cycle the 5 gallon hex in 13 days. Normally it can take 30+ days to cycle a tank and the Nitrite phase is supposed to last the longest (usually 14 days, but some people have got stuck in this phase for 20+ days). I believe that the introduction of Septo-Bac into the tank shortened the Nitrite spike so it was almost the same number of days as the Ammonia Spike. 7 days for the ammonia spike vs 6 days for the Nitrite spike.

From: Tom
Hi, I'm thinking about getting a 60 litre tank with 5 tiger barbs and 5 green tiger barbs. How should I go through the cycle and when should I add the fish. Thanks. tom

Re-read the options above for getting the cycle started. Once you are getting nitrates and no readings of ammonia nor nitrites you can safely add fish to your newly cycled aquarium.

From: Kelly
I do use the biospira and it works very well.
Thanks for the comment on bio-spira. I've heard many successful reports from hobbyists using this product toutralize the chlorine. Once your biological filter is established, add a few more fish and so forth. Also note that when cleaning aquarium decor, use the same water from your tank as the normal tap water will kill all good bacteria. Happy fish keeping and remember to do at least a 20% water change weekly.
Indeed, the old fashioned way of doing the cycle. There are better, quicker more humane ways (fishless) to do it nowadays though. Also, just want to add that letting water sit for 2 days may remove chlorine, but what about chloramine? To our knowledge, you still need a water additive to remove or neutralize chloramine.

From: Mike - Read Up on the Aquarium Cycle
Okay, I read this too late... Bought and set up a 20 gallon tank, let it sit for 4 days with filter running. Then added 6 guppies, 6 neon tetras, 2 swordtails, 2 panda corys and 2 Danios. Over the course of 3 days. Lost a few guppies and one tetra on days 8 and 9. Could this be related to the cycle that I was ignorant about?
Mmm, most likely because the cycle is kicking in gear with the build of ammonia and nitrites which can be very harmful to fish. That's a lot of fish to add all at once. Ideally, you want to add a couple at a time to avoid taxing the cycle. Adding a few at a time allows the beneficial bacteria time to catch up to the increased bio-loads being introduced into the tank. I know it can be very difficult to exercise patience, but that really is the best advice when going through the cycle and stocking a new fish tank.

From: Cassie - Confused about the cycle
I'm so confused! I have a 5 gallon U.S. tank and I've been using it for like a year. I just cleaned everything out using tap water putting everything back in to start fresh because my fish died. I'm doing a lot of research so I can get everything right but I'm not sure what to do about the cycle. Could you please tell me in steps what to do!
Hmm, ok. I'll try to break it down into steps:
  • Fill tank with dechlorinated tap water or pre-mixed saltwater for saltwater aquarium keepers.
  • Determine your method of starting the cycle. Fishless is the quickest and most humane way. For freshwater tanks, get some Bio-spira and add it to the tank. For saltwater tanks, live rock is the way to go. As the rock cures in the tank it will cycle your tank.
  • Test the aquarium water with your test kits for ammonia, nitrite and nitrates.
  • Once you start getting nitrates and no nitrites you can safely add fish. If you're using live rock in saltwater tanks you may not see any of these (ammonia, nitrite and nitrate). If you've had the rock in the tank for several weeks/months you can assume that it's safe to slowly begin stocking the tank.
  • That's it!

From: Cory - Cleaning tank during cycle
Another common problem in the cycle stage is people over clean their tanks during partial water changes and maintenance. If you use the food or fish starting methods, its very important not to disturb the media bedding to allow beneficial bacteria cultivation and growth. Media is the largest source for this growth and over cleaning in the first few weeks can diminish this cultivation causing an elevation of ammonia and/or nitrites.

From: Richard - Doing things the hard way
I just bought a 10 gallon tank and interested in making this a hobby but as I've read everyones comments I've made lots of mistakes. First off, I bought 2 plecos, 2 gold fish and a crayfish and I put them into the tank as soon as I set it up. The water has been pretty cloudy for 2 days and I'm thinking about getting the biospira. Is there anyone who can give me advice on how to have a successful tank. I have the filter, air pump, tap water conditioner and the fish food that I've been feeding. How long does it take for the water to clear? And how do I know if its cycling properly.
Hi Richard - Ok, I think you need to slow down just a bit and read up on the cycle, setting up a tank, research the fish you'd like to keep (look into the smaller tetras or barbs) in a 10 gallon tank and then read some more. The plecos have to go back to the store or you need a much larger tank asap. The goldfish won't do as well with tropical fish and they are heavy waste producers and need at least 20 gallons a piece. They crayfish is another story altogether. The cloudy water could be the bacteria bloom which could be indicative of the beginning stages of the cycle. If you're only interested in keeping a smaller tank like the 10 gallon, look into keeping much smaller species and get that bio-spira too.

From: Chickadee - Tried Two Different Methods
I have used two of the above methods to cycle tanks. I used the Ammonia method and liked the results I got because the tank stayed nice and clean without the mess that fish or fish food would have made but since I was introduced to Bio-spira, my method was set. My one word of advice is that you do not add the Bio-spira and wait for a long time to add the fish. I shake the pouch, pour the Bio-spira in the tank and add the fish. Just that quick. The Bio-spira needs the ammonia that the fish produce to start having something to nourish itself. Do not check the parameters for 7 days or you will make yourself crazy. You will have an ammonia spike, a nitrite spike and some nitrates as it does do a cycle in that 7 days but it does not hurt your fish. After 7 full days do a water change, not before and you will be so pleased with the lovely cycle you have achieved. The fish will be healthy and the tank will be fine.

From: Dan - Using Bio-Spira to Cycle
I have used both freshwater and saltwater bio-spira. I feel that the product helps, but will not "instantly" cycle your tank. The amount of fish I lost when I used in in freshwater (2 out of about 10) was not worth just waiting for it to cycle in my opinion. I used it a few times for salt, and I still waited to put fish in the first time, but I just tried it again today when I had a serious problem with a newer tank and I was worried about my damsels... The nitrites went from 1.5 ppm to 1 ppm and I went from having no nitrates to 1 ppm in just a few hours. It appears to have significantly sped up the cycle. We will see what happens, but I feel it did help.

From: Will - Using Bio-Spira
I tried fishless cycling a 75 gallon tank by adding pure ammonia. After a few weeks without observing any changes in the levels, I lost my patience and added bio-spira. Within 48 hours, I went from 4 ppm ammonia to 0. I never detected any nitrites. I was able to add a full load of fish without any problems at that point, although I did keep a very close eye on the water parameters for the first few weeks.

From: Tyler - Freshwater Bio-Spira Discontinued?
Just to acknowledge people using bio-spira, I believe that freshwater has been retired but Saltwater is still on the market.

From: Kyle - another way to cycle a tank
I haven't ever had to cycle a new tank for more than two weeks... here's what I do:
A week before I am going to get a new aquarium, I buy a filter that cycles half the new tank's capacity and get that one started on my existing aquarium (it helps clean up the current aquarium and gets bacteria colonies started). Then, when I get the new aquarium, I do a water change on my existing one with a filter on the other end to catch the detritus and just get the cycled water. I then put that water in the new tank and then fill it up the rest of the way, start the new filter and move the one that has been on the existing aquarium as well. Then, I use the API product for boosting the cycle, and put hardy fish like tiger barbs in the new tank and do not change the water for a month. It's never failed on me.
Thanks Kyle. Just be sure to test for ammonia and nitrites before subjecting any fish to the water, even hardy fish like the tiger barb.

From: Ron
I had a 16 gallon aquarium but recently upgraded to a 29 gallon. I used the gravel from my old tank and put it in the new one but because of the larger tank size I needed a new filter. Even with that the whole cycle took 3 days and my ammonia spiked after just a day and now my nitrites and nitrates are all good now too. This is a method hard for a lot of people especially those new to the hobby, but it worked really well and really fast.
Yep, good point. This is another way many people with MTS get their new tanks up and running.

From: Mike S. - Odor from tank
Wow, I never knew about this cycle process. We had three gold fish living in our pond. I wanted to transfer them to a fish tank inside because of the algae build up on the fountain portion of the pond. Our gold fish grew to be about 10 inches long and felt I needed a bigger tank. I of course set everything up, filled it and didn't hesitate to put the fish in. In addition, my kids were so excited they wanted more fish, so I put 4 addional fish in at the same time. Everyone seems to be doing ok except for the tank. I will need to finish the cycle process with the fish in there, but I cannot seem to control the odor comming from the tank. Is this a result from the cycle process taking place?
The odor is most likely the result of too many fish in the tank with no bio-filtration capabilities. You can fix the odor by using activated carbon in the filter. Stay on top of the partial water changes throughout the cycle so that your fish survive. Monitor the ammonia and nitrites every couple of days. You don't mention the aquarium size but hopefully it's big enough to support the fish you have in it. You could use some of the filter media from the pond filter on the inside filter to "seed" it with the bacteria necessary to jump start the cycling process.

From: Tom - using ammonia or raw shrimp
Best way to do a Cycle is either 100 percent Pure Ammonia (Shake the bottle and if you see suds then don't use it, usually the cheapest ammonia is the one to buy) or an uncooked raw shrimp. Take out your charcoal as this will prolong a cycle. I've seen tanks fully cycled this way in 10 days.

From: Rebecca
Reading these guidelines will help me greatly in starting up my own aquarium. I never knew all of the necessary precautions that are involved in setting up a tank. I now know how to make sure that my fish are healthy and happy!

From: Alec - good advice
When starting an aquarium, always make sure that the tank size is large enough for all of the fish, Test the water regularly and make sure you buy only healthy fish. It is critical that you buy fish that have not been exposed to any diseases. If the fish you plan to purchase look or behave strangely, or there are dead fish in the tank, make sure to refrain from purchasing those fish.

From: Gaby - Increase Temperature to speed up cycle
Increasing the temperature of your aquarium to 80-82 degrees Fahrenheit really does help speed up the Nitrogen Cycling process!

From: Grace H.
Make sure the cycle is complete before adding fish. Using tetra safe start might be the best bet because you have no risk of contaminating the water, and the fish are not at all at risk. Make sure you monitor the tank REGULARLY and record your results to keep track and make sure everything is in order.

Video - Cycling With Fish In Tank

Watch the video below for more details on cycling with fish!

Useful products mentioned in the video:
Tetra SafeStart Plus - beneficial bacteria in a bottle.

Digital Thermometer - for easily measuring tank temperature.

Ceramic Rings Filter Media - filter media for the bacteria to grown on.

Freshwater Master Test Kit - essential for monitoring the cycling process.

Still have questions?
Jump on the forum and ask your question!
Go here: Aquarium Nitrogen Cycle Forum

Article References :
School teachers please check out the following cycle curriculum guide that you can use to teach your students about the nitrogen cycle: There's Something Fishy