When is the Nitrogen Cycle Complete in Your New Aquarium

When I got my first aquarium I was extremely eager to add in fish, but a friend told me to wait until the tank was fully cycled. I did not know when my tank’s cycle would be done, so I did some research. This is what I found.

When is an aquarium fully cycled and safe for your fish? A fish tank is fully cycled when the colony of beneficial bacteria is strong enough to convert ammonia first in to nitrite and then into nitrate. The ammonia that is produced by fish waste, rotting leftover food and decomposing organic waste. While ammonia and nitrite are highly toxic to fish in low concentrations, nitrate is much less so.

Building a colony of beneficial bacteria takes some time. Also, what exactly do you want to see when you measure your water parameters? These are, amongst other, some things that I will explain in the rest of the article.

Why cycle an aquarium

In short, the aquarium nitrogen cycle is what allows your fish to live in your aquarium. The main factor in the cycle is a colony of bacteria that are crucial in the cycle of yor tank. These bacteria take ammonia that comes from fish waste and rotting matter, and convert it to nitrite and finally to nitrate.

This is important, because both ammonia and nitrite are toxic in low concentrations. Way more toxic that nitrate in the same concentration.

If a tank is not cycled, the produced ammonia that is diluted in the water of an aquarium can not be handled by your tank, and will therefore poison your fish, causing ammonia stress. Too much ammonia will eventually result in the death of your fish.

Desired water parameters when tested

  • 0 ppm (parts per million) ammonia
  • 0 ppm nitrite
  • nitrate should build up over time, meaning that ammonia is immediately broken down into nitrate and finally into nitrate

To know for sure that your aquarium is perfectly cycled, you should monitor the water parameters for a couple of days. You can do this by using a liquid based test kit. It is best to not use test strips for this, because they can not show concentrations accurately. If you do not have one yet, a good possibility is for example the API master test kit. You can check the latest price here on Amazon.

An aquarium is usually cycled after 4 to 6 weeks. In the last week, measure your water parameters every day. If you read 0 ammonia and 0 nitrite during the entire week, while nitrate is slowly increasing, your tank is cycled.


Some possible test results are given below, together with a brief explanations of what these results imply. Below is the first measurement.

  • 4 ppm ammonia
  • 1 ppm nitrite
  • 0 ppm nitrate

With this measurement, you can deduce that your tank is only in the beginning stages of the cycle. There are already some bacteria that have broken down a small part of the ammonia into nitrite, but the nitrite has yet to be converted to nitrate.

With these measurements, your water is definitely not ready for fish. Let us move on to the next measurement.

  • 2 ppm ammonia
  • 4 ppm nitrite
  • 8 ppm nitrate

Seeing these measurement results, it is safe to say that your aquarium has continued cycling and everything is going according to plan. Some ammonia has already been converted to its final state: nitrate.

Is the water already fish ready? No, but it is getting there. The bacteria will need a bit more time to develop and to convert all the ammonia and nitrite into nitrate.

  • 0 ppm ammonia
  • 8 ppm nitrite
  • 14 ppm nitrate

If you read measurement similar to the results above, you know that the cycle is almost complete. All the ammonia is already converted, but there is some nitrite left that is toxic to your fish. Let your tank be and measure again in one or two days to see the progress.

And then, finally, the ideal measurement:

  • 0 ppm ammonia
  • 0 ppm nitrite
  • 17 ppm nitrate

Sidenote: the nitrate concentration is arbitrary and can vary

When you read this measurement, you are in the green. Your tank is cycled. There are a couple of things you have to keep in mind going forward.

  1. If you measure 0 ppm ammonia and 0 ppm nitrite, you aquarium is only ready for fish if this is because the bacteria are set. If you add water for the first time and immediately measure, you probably will not find any ammonia/nitrite in your tap water.
  2. Do not immediately add a lot of fish at once! This is important, because your bacteria colony is not extremely strong from the get go. If can handle some ammonia (bioload), but not too much. If you add too many fish, chances are that your tank can not handle all the ammonia produced, and the concentrations will rise again.
  3. After you added some fish, measure your water parameters once a week to see if everything is going according to plan. Make sure there is no ammonia or nitrite present when you measure.

Actions that can slow down or stop the cycle

  • a water change that is too big (> 50%)
  • filter stopped pumping water past the bacteria
  • all ammonia converted to nitrate (no food left for bacteria)

You need to be aware that the aquarium nitrogen cycle can be stopped, slowed or reset completely. This happens when the beneficial bacteria housed in your filter are (partially) killed, and they need to start growing again.

This can happen when you do a water change that is too big (> 50%), which will expose parts of the bacteria to air. This will take away their food source for too long and they will die off.

Most of the times this will only slow down the cycle.

Another reason is a malfunction in the filter pump, which stops the flow of new water that is pumped through your filter. If there is no water that provides the bacteria with ammonia, they die.

Finally, the chance that there is no ammonia left at all in your water. This one is way less likely, but can happen if you do a fishless cycle without adding a source of ammonia.
The bacteria definitely need some ammonia to feed and grow.

Fishless v.s. classic cycle

With the previous information in mind, I want you to know that there are different ways to cycle (build a bacteria colony) an aquarium. In the more traditional way, fish are used as an ammonia source, and in the fishless cycle food or liquid ammonia is used as the ammonia source.

Classic aquarium cycle

In the past, aquarium keepers would add fish to their aquarium right from the start. These usually were “starter” fish that were hardy fish to help start the aquarium cycle. They were the ammonia source needed to feed the growing colony of beneficial bacteria.

You can imagine what happened when ammonia levels rose and there were no bacteria to break it down into something less toxic: these fish would die.

If the fish would not die, they were exposed to a big amount of stress anyhow.

As soon as the cycle was complete, the hardy starter fish were swapped for the desired fish that the keepers intended to keep in the fish tank from the start.

Fishless aquarium cycle

In the last decade, the fishless cycle has gained popularity amongst aquarium hobbyists. This is also the method I use, prefer and recommend because it does not involve fish stressing and dying.

When you set up an aquarium, it does not have a colony of bacteria to break down ammonia, so it needs to grow one. To start this off, there needs to be some ammonia in the tank.

Now there are two option. The first one is adding ammonia that you can buy in the stores as a cleaning product. Make sure there is no added substance! It needs to be pure pure pure ammonia. This is something I can not stress enough because a lot of cleaner ammonia has something else mixed in it.

The second option is what I do, and that is adding just a little bit of fish food from the start. My local fish store taught me this when I bought my first tank, and it worked for me.

What you do is you add some fish food, just a little bit, which will rot and break down into ammonia. They you are patient and you regularly measure the parameters.

Related Questions

How to instantly cycle a new aquarium? Instantly cycling a new aquarium is possible when you can provide is with a way to convert ammonia into nitrite and finally into nitrate. To do this, you can add “cycled media” which is filter media taken from another filter that has been running in another aquarium

How to speed up the aquarium nitrogen cycle? Speeding up the cycle is something all enthusiastic fish owners desire. It is possible by adding a starter bacteria/product that is available on the market. If you are interested in something like that, take a look at this product called Tetra SafeStart on Amazon.

Bart Sprenkels

I have been keeping multiple aquariums since I was 18 years old. Just like many of you, I started with two goldfish but quickly learned they were not suitable for aquariums. Later, I switched to a tropical community tank and I also have two pet musk-turtles in a bigger aquarium. You can read more about me here.

Recent Content