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Aquarium Cycling – An easy and complete guide

aquarium cycling guide cycle

Aquarium cycling involves using the Nitrogen cycle to our advantage. The Nitrogen cycle provides us with a waste management system to keep our fishy friends safe and healthy. Understanding the Nitrogen cycle is really important in fish keeping as it can help us to detect issues easily and effectively. Here is my easy to follow guide to a cycled aquarium.

Aquarium Cycling

If you’re new to fish keeping, then you have no doubt heard the term ‘Aquarium Cycling’ or something similar and you’re probably either already confused or you’re trying to learn what that means.

This article will cover the bases of aquarium cycling in an easy to follow step-by-step manner, so by the time you have finished reading this, it will all make perfect sense……hopefully!


aquarium cycling

What is Aquarium Cycling?

Aquarium cycling is all about establishing a kind of mini eco-system within your aquarium.

Imagine now that you are in your fish’ place, living in a glass box. The glass box is home, you eat there, sleep there and, well, poop there. I think we would all agree that if that was us, living in that glass box of water, life would become pretty unpleasant the moment we went to the bathroom.

As humans, we flush our waste away and forget about it. But our fish don’t have that luxury. When they go to the loo, it doesn’t go too far away and they are stuck in there with it.

Now, it would be pretty impractical to be hoovering out your fishes business every time they went to the toilet, so what we need is a way of breaking that waste down to remove it before it poisons our fish.


Luckily for us, nature is amazing and has given us a waste management system free of charge. Bacteria!

As humans, we often think of bacteria as a horrible, nasty, harmful little thing and we avoid it as much as possible. But some bacteria is really useful and it can do amazing things.


You may now be wondering, if bacteria already do this for me, why have I bought a filter?

The bacteria that we need will live mostly in your filter. The filter will help to remove fish waste and uneaten food from the tank, keeping your water clean. The bacteria in the filter will then break it down and eat it. Yummy!

So technically, we are not cycling the aquarium, we are cycling the filter.

More on filters later.


So, what is Aquarium cycling? In short, Aquarium cycling is a process we use to cultivate this beneficial bacteria that will keep our fish happy and healthy.


What do I need for Aquarium Cycling?

Aquarium Cycling requires a few things to complete. I’ll go through each item in a moment but the first and most important thing you will need is patience.

It can be a lengthy process to correctly cycle an Aquarium (often up to 8 weeks) and this regularly leads to new fish keepers buying fish too soon, only to end up with a box of dead fish. There is a shortcut though, which I will explain later.

It is possible to cycle an aquarium with fish in, but I don’t recommend it for a few reasons:

  • Fish in cycling can be very harmful to the fish and is pretty inhumane.
  • Fish in cycling is much harder work.
  • Fish in cycling usually takes longer (if done correctly).

More on Fish in cycling later.

Many Aquarium shops (most often big brand shops) will happily sell you a fish tank and tell you to come back in a few days for fish. They will often sell you a bottle of bacteria and tell you that it will cycle your tank instantly.

Unfortunately, this isn’t the case and often leads to loads of problems along with dead fish. Funnily enough, once your poor fish have died, you will want some more, quids in for the fish shop!

Bottled bacteria is a great idea and it may speed cycling up a little, so use it if you wish. But it will not cycle it in a few days, there are some other sure-fire ways to ‘quick cycle’ an aquarium, which I will cover shortly.


What else will I need for Aquarium Cycling?

In order to cycle your aquarium, you will need to add these items to your shopping list if you don’t have them already:

  1. An aquarium with an appropriately sized/powered filter. Your filter is where your bacteria is going to live. It needs to be powerful enough for the size of the aquarium you are setting up so that it is able to keep your aquarium clean.
  2. Some appropriate media to go in that filter – See our filter media setup guide HERE. Filter media is what your bacteria will live on.
  3. A source of Ammonia. Ammonia is the toxic compound that is released from the break down of your fishes waste. This is what the bacteria feed on, so we need to provide some in the absence of fish waste in order to grow some bacteria. For this, we need an Aquarium safe ammonia source (Link below). Alternatively, add fish food every day.
  4. A liquid dropper Aquarium test kit. Liquid test kits are far more accurate than strips. The best commonly available kits (in my opinion) that don’t break the bank are API and NT labs (Links below).




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Water testing kits come with instructions on what each test is for but for a bit more info from us, check our short guide to water chemistry.

For a guide to using a water testing kit, find a demo on the API master test kit by Kaveman Aquatics HERE.

That’s it! But what you will also benefit from is some understanding of the Nitrogen Cycle…….


The Nitrogen Cycle – Aquarium cycling

The nitrogen cycle is a natural process of breaking down nitrogenous (containing nitrogen) compounds, converting them into other compounds. This happens constantly in nature and we can utilise part of this cycle in our aquariums.

Make sense? – Probably not….yet.

So, when our fish go to the loo their waste contains ammonia, just as ours does. Ammonia is a toxic nitrogenous compound and can be lethal to our fish even at pretty low levels.

So this ammonia needs to be broken down into something less poisonous.

For this, we use our first culture of bacteria, for ease we’ll call this Bacteria-1. Bacteria-1 feeds on ammonia, breaking it down into nitrite.

Nitrite is less toxic than ammonia but still deadly at fairly low levels. So we need that to break down too.

For this, we rely on other bacteria, which we will call Bacteria-2. Bacteria-2 feeds on nitrite breaking it down into nitrate.

Nitrate is much less toxic and requires much higher levels to kill or harm our fish.

Nitrate is very difficult to break down in the aquarium as it requires anaerobic (without oxygen) conditions which are very difficult to create in our well-oxygenated tanks. So to remove this we do regular maintenance and water changes.

However, plants will use nitrate as food. So after cycling is complete, adding live plants to your aquarium is a great idea!


pH shock aquarium cycling cycle


Aquarium cycling – getting started

To cycle your aquarium you will first need to set it up. At this point, you will need your tank filled with dechlorinated water, a filter, a testing kit and a source of ammonia.

At this point, everything else is optional really, but it can be handy to set the tank up as you want it to save the faff later (plus it’s nicer to look at). A heater can be useful too as warm water encourages bacteria growth.


First off we want to grow a colony of Bateria-1. Bacteria-1 feeds on ammonia, so we need to add some to the aquarium. If you’re using the ammonia linked above, there should be some instructions on how much to use for your aquarium size. (If you’re using fish food, just throw some in, it doesn’t matter how much at this point).


Dosing Ammonia

We want to dose the aquarium to around 4 parts per millilitre (ppm) ammonia, this is the first use of our testing kit. 4ppm ammonia is a great starting point for an aquarium but don’t worry if you go over.

There is a common myth that anything above 4ppm ammonia will stop aquarium cycling. This simply isn’t true. Does too big a buffet stop people from eating it?

No, the bacteria will still eat ammonia. So you don’t need to be overly accurate.  (If you are using fish food, you will not be able to measure this yet. The fish food will take a few days to break down and produce ammonia).

From here, you will need to test the aquarium regularly (daily is ideal) to ensure that the level of ammonia remains around 4ppm. If it starts to drop, top it up. Ammonia is the food source for the bacteria we are trying to grow, so we need to keep it in plentiful supply.

This is where your most important ingredient needs to start working:



We are now waiting for our test kit to read some level of nitrite. Detecting rising levels of nitrite means that we now have a growing colony of Bacteria-1. It is eating up the ammonia and producing nitrite as waste.

Many new aquarists have an idea that this is an overnight process, but it may be 2 weeks before you see a reading of nitrite. Sometimes longer.


We have Nitrite!

Once you have started to detect nitrite in your tests the waiting continues, so your main ingredient of patience needs to keep working. But this is great news and you are well on the way to a cycled aquarium.

Nitrite is eaten by Bacteria-2, so we are now providing food for our second colony of beneficial bacteria.

What you need to do here is everything you have been doing up to now. This means regular tests and keeping the ammonia topped up. As your colony of Bacteria-1 continues to grow, the ammonia level will drop quicker and quicker.

If ammonia runs dry, then Bacteria-1 stops producing nitrite, which means that there is no nitrite for Bacteria-2 to eat, so any Bacteria-2 stops growing. This will only slow things down. So keep that ammonia level up!

Whilst testing, we’re now looking out for our nitrates starting to rise. Rising nitrates is an indication that we have established a colony of Bacteria-2 that is starting to eat up those deadly nitrites.

Great news!



Nitrates are the end of the cycle in most cases, so a reading of nitrates on your tests is excellent news.

You should now be seeing your ammonia levels dropping daily, eventually to 0 within 24 hours, this is exactly what we have been aiming for.

But if it isn’t quite doing that yet, don’t worry. Keep going as you have been until it does, it won’t be too long now.

Getting to this stage can often take up to 8 weeks, so patience is critical throughout. The important thing to remember is to keep your ammonia level up. This ensures a constant supply of food to keep that bacteria growing.


Extra’s to keep in mind

  • Bacteria 1 and 2 both require oxygen – well-oxygenated water encourages bacteria growth.
  • Warm temperatures (25 – 30 C) are ideal for bacteria growth.
  • A pH below 6 can severely inhibit bacterial growth.
    • There is no need to do water changes until the filter is cycled.
  • Live plants absorb ammonia, nitrite and nitrate as food. Whilst this can be useful it can prevent a new tank from cycling. So it’s best to wait until cycling is complete to add them.


Common issues whilst Aquarium Cycling

There are a few issues that almost always occur whilst cycling, so here is a quick overview of what they are:

  1. White cloudy water – white clouding in the water is normal, this is bacteria multiplying. Don’t panic! This is completely normal and it will rectify itself as the bacteria finds an equilibrium (balance).
  2. Algae – Algae is common when cycling. With ammonia and nitrite in abundance, the algae will feed on it. Believe it or not, this is a good sign. You can minimise algae by keeping the light off. Things will calm down as your tank matures but for more info see our Algae Infestations article. Leaving the light off will greatly reduce algae!
  3. Nothing is happening!!! – This is where you need to rely on that all-important patience. Something will happen, you just need to wait.
  4. Water changes. Some people will tell you to do water changes. Some will tell you not to. There is only a need to do water changes while cycling if your pH drops to 6 or below. This means that the KH value of your water is depleted and the pH is not buffered.

When either of the first two issues occurs, the temptation to clean the tank can be overwhelming. But please resist the urge to clean the tank too soon as you could reset the cycle, particularly if you clean the filter. Both of these issues will be greatly reduced by themselves as your filter develops a colony of bacteria.


How do I know when my Aquarium is cycled?

This is a regular question. How do you know when it’s cycled?

If you top your aquarium up to 4ppm of ammonia and get a test reading of 0 ammonia, 0 nitrite and rising nitrates within 24 hours for 3 days in a row then your tank is cycled.

In an established aquarium, ammonia and nitrite should be undetectable. Nitrate should be kept as low as possible by weekly water changes, or more frequently if needed.

What to do now?

Once your tank is cycled, it is almost ready for fish.

But don’t go mad!

Your tank has cycled to deal with 4ppm of ammonia in 24 hours, or whatever level you chose to cycle it at. So if you add too many fish, they may produce more ammonia than that and be poisoned.

The size of your aquarium will dictate how many and what type of fish you can add in total. Adding a small group of small schooling fish, like Tetra, or a single larger fish is a good way to start.

From here, keep testing the water to ensure that your bacteria is coping with the load and slowly add more fish. Leave at least a week in between each new addition to ensure the filter is coping well.

If you arent getting any fish for a few days, or longer, you will need to keep adding ammonia to keep your colonies of bacteria fed. If you stop adding ammonia and don’t add any fish, the bacteria can go dormant, or even die, and we’re back to square one again.


Before adding fish

Before adding fish you will need to start maintaining your tank. During cycling, your nitrate levels should have been rising and are probably pretty high by now.

If you’re using the API test kit then nitrate should always be below 80ppm, ideally as low as possible though.

If you’re using an NT labs test kit then nitrate should always read under 20ppm, but ideally as low as possible.

(The reason for the different readings is due to the way these two kits measure nitrates, I will write this up in more detail and link it here once I have – HERE is the link).

Bear in mind that you are about to add fish. When you do, nitrate will climb, so your readings need to be much lower than the figures above before adding them.

To achieve lower nitrate levels, you need to do a water change.


aquarium maintenance aquarium cycling cycle


Your Filter

Once cycled, your filter is the thing that will keep your fish alive. So it needs to be looked after. So does the bacteria inside it, without which it is pretty useless.

A freshly cycled filter doesn’t need cleaning.

Aquarium shops and filter manufacturers will tell you that you need to change your filter media every month or so. This isn’t necessary.

Changing filter media = bye-bye bacteria.

So, keep your filter media until it is literally falling to bits, then change half out at a time at the most. If you change half at a time, at least half of your bacteria colony is kept. It will then need time to re-grow on the new media (about a month), then change the other half.

Good filter media will almost never need changing. Solid bio-media should last forever.


Never wash your filter media in tap water. The chlorine in tap water is there specifically to kill bacteria and that’s exactly what it will do. Leaving your filter uncycled and your fish in danger.

For more on filter media maintenance, see our quick guide to aquarium cleaning.



JBL E1902 filter review aquarium cycling


Fish in Cycling

Fish-in cycling is cycling your aquarium, or filter, with fish living in it.

As previously mentioned, I don’t recommend it, certainly not on purpose.

Before we start, it is worth noting that there is no guarantee that fish will survive fish-in cycling. But this guide will give you a fighting chance:

Fish-in cycling is now the ‘old way’ of starting a fish tank up. It is a little outdated but there are still many aquarists who cycle a tank this way. Why? Who knows?

In some cases though, you may find yourself accidentally fish-in cycling, so it’s handy to know what you need to do if that happens.

Accidental fish-in cycling could occur for many reasons, a few common ones are:

  • Overcleaning the filter.
  • Adding too many fish in a short space of time.
  • A dead fish in the tank causing a massive ammonia load.

All of the above can cause ammonia to rise if your filter cannot deal with the amount of ammonia produced.

As mentioned already, ammonia and nitrite are highly toxic. Fish can deal with some ammonia and nitrite, but the bacteria they cause in the water column can often kill fish pretty quickly.

Many aquarists recommend using a water conditioner such as Seachem prime for fish-in cycling. This is because Seachem state that their conditioner can detoxify ammonia and nitrite.

Unfortunately, there is no evidence of this but there have been several tests done that prove it doesn’t work. So these types of water conditioners cannot be relied upon over hard work and dedication to keep your fish alive.


Fish in Cycling – Ammonia and Nitrite levels

As it is toxic and causes an increased number of bacteria (bad kind) in the water, with fish in the aquarium the ammonia level needs to be kept as low as 0.25ppm until the filter has developed enough bacteria to reduce it to zero by itself.

This means that daily water changes are usually necessary to keep it that low.

Nitrite needs to stay under 1ppm. This is also controlled by water changes.

You will need to keep these levels this low until your filter cycles, or re-cycles, depending on how this situation arose.

If the ammonia spike was caused by a deceased fish, then you may well find that removing the body and one large water change brings things back in line.


Fish in Cyling takes longer!

The cycling process for fish-in cycling remains the same as above, but keeping such low ammonia levels can mean it takes much longer to complete cycling.

This is because the low levels of ammonia can only support a smaller colony of bacteria, then it has to grow slowly.

Without an established filter, fish are more prone to diseases such as Ich/Whitespot too, so it can be a very hard process for any fish involved.

Keep a close eye on them for signs of illness or disease throughout and treat anything that you spot as soon as possible.

I’m sure it goes without saying but you do not need to add ammonia for fish-in cycling, your fishes pee, poop and food will do that for you.


Aquarium cycling shortcuts!!

As promised, I have a couple of shortcuts that could see you saving weeks of time waiting for your cycle to establish.

Below are two ways to drastically cut down the time it takes to cycle your filter:

  1. Used filter media: taking filter media directly from an established, healthy filter will immediately add live bacteria to the new filter when placed inside it. The more used media you add, the quicker it cycles. If you add enough (fill the filter with it) then it can be cycled instantly.
  2. If you don’t know anyone willing to give you some used filter media then see if anyone will part with the horrid brown gunk from their filter instead. As disgusting as it may be, it is filled with bacteria that will significantly speed up the cycling process.

These are the only real, reliable ways to speed up aquarium cycling. Bottled bacteria may help but I have known it to cause issues in the past, sometimes creating false readings when testing aquarium water.


Happy cycling!! If you have any questions, ask away in the comments or in our forums!


Setting up a new aquarium? Have look through our list of recommended products in our ‘Shop’: HERE


aquarium cycling



Frequently asked questions:

What is Aquarium cycling?

Aquarium cycling is a process used to build colonies of beneficial bacteria within the aquarium filter. This bacteria breaks down toxic waste and keeps the water clean and healthy for your fish.

How can I speed up Aquarium cycling?

Adding live bacteria to your filter can increase the rate that it cycles. The best ways to do this are to add mature filter media or to add the brown 'gunk' that has built up in a mature filter to your new filter. This adds live bacteria which then multiply.

How long does it take to cycle an Aquarium?

Fish-less cycling can take up to 8 weeks to complete from scratch. Fish-in cycling often takes longer.

Is my Aquarium cycled?

If you have added ammonia to your aquarium each day and your test shows 0ppm ammonia within 24 hours of ammonia being added for multiple days, then your aquarium is cycled.

I have fish in my tank but tests show ammonia and/or nitrite, what do I do?

If you have purchased some fish and are detecting ammonia and/or nitrite, then the aquarium is not cycled. At least not to the current bioload. This is called Fish-in cycling and will require regular (daily) water changes to keep ammonia and nitrite low.

Aquarium cycling - Nothing is happening, have I gone wrong?

If you are cycling your aquarium using our guide and nothing has happened yet, don't worry. It takes time to cycle an aquarium and the main ingredient is patience. Hold out a little longer and it will get there.

I have nitrates but I haven't seen any nitrites?

Occasionally you may see nitrates appearing without seeing any nitrites on your tests. This is a good sign and means that some nitrite has been converted to nitrate without you even noticing. As your bacteria levels increase and more nitrite is produced you will start to see it on your tests before it is converted to nitrate.



Aquarium science



About the author

James @Seriously Fishy

I am a fish keeping enthusiast with over 20 years experience. I currently keep American Cichlids (CA) which are my favourite fish to keep so far. I started Seriously Fishy as I noticed a large volume of people on various web pages looking for help with Aquarium basics. I created the first Seriously Fishy book to solve the issue in 28 pages, that led to the Seriously Fishy UK fish forum and blog.

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