The Aquarium Nitrogen Cycle

This 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!

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 Nitrogen Cycle. This very important cycle is 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. Check out the aquarium water chemistry page (on the left) for more information on these terms.

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 understand this process. Learning about this process will help you to be successful in keeping fish and it should definitely improve your chances when keeping tropical fish. The best way to monitor the nitrogen cycle is to purchase an aquarium test kit that will test for ammonia, nitrites, nitrates and ph.

Test your aquarium water every other day and write down your 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 your tropical fish.

Photo Credit: Ilmari Karonen

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. 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 tropical 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. In 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.

Getting The Nitrogen Cycle Started
There are two ways to get the aquarium cycle started, either with fish or without fish.

Starting The Nitrogen Cycle With Fish
This is not the preferred way to get the nitrogen 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 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. There is a better way. Read on, young grasshopper.

Starting The Nitrogen Cycle Fishless
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.

Option 1:
Using Fish Food
Drop in a few flakes every 12 hours. As the food decomposes it will release ammonia. You will have to continue to "feed" the tank throughout the process to keep it going.
Option 2:
Use a small piece of raw fish or a raw shrimp
Drop a 2 inch by 1 inch chunk of raw fish or a raw shrimp into the tank. As it decomposes it will release ammonia into the tank.
Option 3:
Use 100% pure ammonia.
Using a dropper, add 5 drops of ammonia per 10 gallons of aquarium water. If you don't get an ammonia reading with your test kit, add some more drops until you start to see an ammonia reading. Keep track of how many drops you've used so you can repeat this process daily. Continue to dose the tank with ammonia until you start to get nitrite readings with your test kit. Once you can detect nitrites you should only add 3 drops of ammonia per 10 gallons of aquarium water, or if you added more drops originally to get an ammonia reading cut the amount of drops used in half. Continue this process daily until you get nitrate readings with your test kit. Do a 30% water change and your tank is ready.
Option 4:
Use gravel and/or filter media from an established and cycled tank
This is the best and fastest way to go. This will seed the tank with all of the necessary bacteria for the nitrogen cycle. "Feed" the tank daily with flake food until you are getting nitrate readings. Depending on how fast you were able to get the gravel and filter media into your tank, you may be getting nitrate readings in only a day or two. There are some drawbacks to this method. Ask your source if they have recently used any copper medications in the tank. If they have and you are planning to have invertebrates in the tank you should probably not use this method. Invertebrates will not tolerate copper. Get a copper test kit to determine if it's safe to use.
Option 5:
Using live rock in Saltwater Tanks
The use of live rock in saltwater tanks has really taken off over the past few years. The reason for this is because it is one of the best forms of biological filtration available for saltwater tanks. The shape the rock is in when you get it will determine how long the nitrogen cycle will take. See step 7 on the saltwater setup page for more information on live rock.
Option 6:
Use Biospira made by Marineland. This product claims to contain some patent pending species of nitrifying bacteria that will cycle your tank in 24 hours. Some of the FishLore forum members have tried it and it sounds like it is legitimate. It is kind of expensive, but if you already have fish in your tank and they are suffering through the cycle, you may want to check this stuff out. 1 ounce of this product is supposed to treat a 30 gallon freshwater tank. There are both freshwater and saltwater versions of Bio-spira. Please let us know if you use this and if it works for you by submitting comments below.

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.

Increase the temperature of your aquarium water to 80°F-82°F (27°C-28°C)
Get some beneficial bacteria colonies. Borrow some gravel from an established and cycled aquarium. If you have another tank with an extra filter you can use it. If you have a really nice friend with an established and cycled aquarium, ask if you can have one of their used filter media. It will be loaded with the good bacteria that we are looking for.
There are products on the market that claim to introduce the beneficial bacteria. For more information, check out this product called Cycle Water Conditioner.
Also check out Bio-spira in option 6 above.

Nitrogen Cycle Tips / 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 airstones 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 nitrogen 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 to start the nitrogen cycle. Thanks for the comment. Mike

From: Dave
I set up my new 55 gallon tank a little over a week ago and at the suggestion of a fellow member of a fish forum tried Bio-Spira to cycle it. After eight days, the tank is done and ready for fish! I strongly recommend this product to anyone confronting the establishment of a new aquarium.

From: Joe - Cycling Aquarium with Fish
The best way to cycle a new tank is "patience". Start off with about five fish and do a 10% water change every other day. Ensure that you add anti-chlorine in the new water or let the water settle for about 2 days to neutralize 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 nitrogen 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.

More Aquarium Nitrogen Cycle Tips

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