You have bought the aquarium. The substrate is in. The filter is running. Perhaps you have planted it, added some wood and arranged the rocks for the fifth time because that one piece still doesn't look quite right.
It looks like an aquarium.
So, can you add fish?
Not necessarily.
Before fish go into a new aquarium, there is an important job happening that you cannot see. Your filter needs to develop the biological community that will process the waste produced once animals begin living in the tank.
That process is usually called cycling the aquarium.
It sounds technical when you first encounter it. Fortunately, the basic idea is fairly straightforward.
What does “cycling an aquarium” actually mean?
Fish produce waste. Uneaten food, dead plant material and other organic matter also break down in an aquarium.
One of the substances produced is ammonia.
Ammonia is toxic to fish. It is also invisible, so a tank containing dangerous ammonia levels can still look beautifully clear.
Fortunately, an established aquarium contains microorganisms that process this waste.
In simple terms:
ammonia → nitrite → nitrate
Different groups of microorganisms carry out these conversions. Ammonia is oxidised into nitrite, which is also toxic to fish. Other microorganisms then convert nitrite into nitrate, which is considerably less toxic.
This biological process is called nitrification.
When we talk about “cycling” an aquarium, what we are really doing is giving those microorganisms time and the right conditions to establish themselves before expecting the system to deal with the waste produced by fish.
Where do the beneficial bacteria live?
People sometimes talk about beneficial bacteria as though they are mainly floating around in the aquarium water.
They aren't.
They colonise surfaces.
Your substrate, plants, wood and decorations all provide surface area, but the biological media inside your filter is particularly important. A properly designed biofilter provides an environment in which nitrifying microorganisms can establish and flourish.
That has an important practical consequence.
Cycling an aquarium is really about establishing its biological filtration.
It also explains why mature filter media can be so useful when setting up another aquarium.
Do I really need to cycle a new aquarium?
Yes.
There was a time when beginners were routinely told to set up an aquarium, add a few “hardy” fish and allow their waste to cycle the tank.
That is called fish-in cycling.
It works biologically because the fish provide the ammonia required by the developing microorganisms. The problem is that the animals themselves can be exposed to ammonia and nitrite while that biological filter develops.
There is a better option.
For a new aquarium with no livestock in it, I recommend fishless cycling.
You establish the biological filter using an ammonia source without putting fish through the process. Fishless cycling is an established method of developing a biofilter and gives you control over the ammonia being introduced without risking fish during the process.
You get to make your beginner mistakes while nobody is living in the tank.
That seems a rather good arrangement to me.
What do you need for a fishless cycle?
You don't need an enormous collection of equipment.
You should have your aquarium fully set up, the filter installed and running continuously, dechlorinated water, an appropriate ammonia source and reliable tests for ammonia and nitrite. A nitrate test is useful too.
If you are using a heater for the eventual livestock, you can have that running as well. Keep the aquarium under normal, stable conditions appropriate for the system you intend to maintain.
The filter should remain running throughout the cycling process.
Nitrification requires oxygen, so good circulation and oxygenation are important for the organisms establishing in the biofilter.
And then there is the final ingredient.
Patience.
Unfortunately, nobody has yet managed to bottle that and sell it at the aquarium shop.
Step 1: Fill the aquarium and treat the water
Set up the aquarium as you intend to run it.
Add substrate, hardscape and plants if you are using them. Fill the tank and treat your tap water appropriately with water conditioner if your water supply contains chlorine or chloramine.
Start the filter and leave it running continuously.
At this point, the aquarium may look finished.
Biologically, it isn't.
Step 2: Give the developing filter a source of ammonia
Without ammonia, the organisms you want to establish have no food source.
In a stocked aquarium, ammonia ultimately comes largely from the animals and decomposing organic material.
During a fishless cycle, we provide the ammonia without putting fish through the process.
There are several ways of doing this. One of the most controlled is to use an ammonia or ammonium chloride product specifically intended for fishless aquarium cycling.
Follow the instructions for the particular product rather than guessing at quantities.
Some aquarists use decomposing fish food as their ammonia source. It can work, but it is slower and much less precise because you have little control over exactly how much ammonia will be produced or when.
For a beginner, I prefer the more measurable approach.
Step 3: Give the biological filter a head start if you can
A new aquarium will develop nitrifying microorganisms naturally, but there are ways of helping the process along.
One of the best shortcuts, when safely available, is mature biological filter media from a healthy, disease-free established aquarium.
That media already contains an established microbial community.
If you already keep aquariums, this is one of the great advantages of having mature filters available. A piece of established sponge or biological media can give a new system a considerable head start.
Don't take it from an aquarium with unexplained illness, parasites or disease. Saving some time on cycling isn't much of a victory if your new aquarium gets something unpleasant thrown in for free.
Commercial bottled bacterial products are another option. They can be useful, although products differ and should be used according to the manufacturer's instructions. Seeding a new biofilter with healthy mature media or a reputable bacterial culture are both recognised ways of accelerating biofilter establishment.
Neither changes the most important rule:
test the water rather than trusting the calendar.
Step 4: Start testing
This is where cycling becomes interesting.
At first, you should expect to detect ammonia because you are providing an ammonia source and the biological filter is not yet mature enough to process it efficiently.
Keep a simple record.
It doesn't need to become a laboratory notebook. A note on your phone is perfectly adequate.
Record:
Date | Ammonia | Nitrite | Nitrate
Test regularly so that you can see how those readings change.
You are looking for a pattern, not merely chasing individual numbers.
Step 5: Watch for nitrite
Eventually something interesting should happen.
The ammonia-processing population begins establishing itself and you start detecting nitrite.
This is progress, but it does not mean the aquarium is ready for fish.
Nitrite is also toxic.
At this stage, new fishkeepers sometimes see ammonia beginning to decline and assume the cycle is complete.
It isn't.
You have only reached the middle.
The second part of the biological filtration now needs to catch up and develop enough capacity to convert that nitrite onwards to nitrate.
Step 6: Watch ammonia and nitrite fall
As the biofilter matures, you should eventually see it processing both stages of waste more effectively.
Ammonia falls.
Nitrite eventually falls too.
Nitrate may begin accumulating as an end product of nitrification.
This is the point where the invisible biology inside the filter is finally starting to behave like the filtration system you need. The characteristic pattern of ammonia rising and falling, followed by nitrite doing the same as nitrate appears, is a useful way of understanding what is happening inside a developing biofilter.
It can be strangely satisfying.
Aquarium keeping does this to people. One day you're buying a colourful fish. Six months later you're staring excitedly at a test tube because it has remained yellow.
Welcome to the hobby.
So how long does cycling take?
This is one of the most common questions beginners ask.
Unfortunately, the answer isn't particularly satisfying:
It takes as long as it takes.
Temperature, pH, oxygen, alkalinity, the amount of mature media used and other conditions can all affect how quickly the biological filter develops. Academic aquaculture literature commonly describes several weeks being required to establish a new biofilter, with considerable variation according to the conditions.
That doesn't mean your home aquarium will take exactly four weeks, six weeks or eight weeks.
It means “I've been running the filter for seven days” is not evidence that an aquarium is cycled.
Neither is cloudy water becoming clear.
Neither is adding a bottle of bacteria.
Neither is the aquarium shop telling you to come back next weekend.
Your water tests tell you what the biological filter is actually doing.
How do I know when my aquarium is cycled?
For a beginner, the clearest signs are:
Ammonia: 0 mg/L
Nitrite: 0 mg/L
and evidence that the system has successfully processed the ammonia source you have been supplying.
If you're carrying out a deliberate ammonia-fed fishless cycle, don't simply stop adding ammonia for a long period, test the water, see zero and conclude that the aquarium is ready.
The useful question is:
Can the biological filter process the waste load being supplied to it?
That is the capability you are trying to establish.
Once your added ammonia is being reliably processed through the system without leaving measurable ammonia or nitrite behind, you have good evidence that the biological filtration is functioning.
What about nitrate?
Nitrate is useful because its appearance can provide evidence that nitrification is taking place.
But I wouldn't teach beginners to treat a particular nitrate number as the universal definition of a completed cycle.
Why?
Because planted aquariums can use nitrate. Your tap water may already contain some nitrate. Different test kits have limitations. And nitrate concentration alone tells you less about immediate cycling readiness than your ammonia and nitrite results.
What matters most before introducing fish is that your biofilter is reliably processing ammonia and nitrite.
Once the aquarium is stocked, nitrate becomes one of several useful indicators that help you judge your maintenance routine.
Should I do a water change before adding fish?
If nitrate or other dissolved material has accumulated significantly during the fishless cycle, a partial water change before introducing livestock is sensible.
Treat the replacement water with conditioner and bring it reasonably close to the aquarium's temperature.
Then test again.
The goal is to begin the stocked life of the aquarium with suitable water conditions and a biological filter that is functioning properly.
And, of course, once the fish are in the aquarium, stop adding your artificial ammonia source. The inhabitants of the aquarium will now provide the waste that keeps the biological filter working.
Can live plants cycle an aquarium?
Plants certainly affect what happens in an aquarium.
Growing plants can take up nitrogen compounds, and their leaves, roots and surrounding surfaces provide additional habitat for microorganisms.
A heavily planted aquarium can therefore behave rather differently from a bare tank.
That doesn't mean I would tell a first-time fishkeeper simply to plant the aquarium and assume everything is fine.
Test it.
If you are new to fishkeeping, knowing what your ammonia and nitrite are doing is far more useful than assuming that the plants have taken care of everything.
Plants are wonderful.
They are not a substitute for knowing what is happening in your water.
What if I already have fish in an uncycled tank?
Sometimes people discover cycling only after they have bought the fish.
It happens.
At that point, telling someone what they should have done last week isn't especially useful.
Your priority changes from establishing the cycle conveniently to protecting the livestock while the biological filter develops.
Test ammonia and nitrite frequently.
If either becomes detectable, partial water changes with properly conditioned water become an important way of reducing the concentration while the biological filter develops. Feed cautiously because excess food increases the waste load. Keep the filter running and don't add more livestock.
Ammonia and nitrite are both potentially dangerous to fish, so this is one situation where regular testing really matters.
If ammonia or nitrite continues rising or fish are showing signs of distress, get experienced help rather than simply waiting for the cycle to sort itself out.
The important thing is to deal with the aquarium you have now.
Can I speed the cycle up?
Sometimes.
The most useful method is mature biological filter media from a healthy established aquarium.
Think about why.
You aren't adding magical “old water”. You are physically transferring some of the biological community that you are trying to establish.
This is also why I am rather protective of mature filter media.
Don't replace all of it because it looks brown.
Don't scrub every filter component until it looks factory-new.
And don't sterilise an established biological filter as part of routine maintenance.
The slightly grubby sponge in your filter may be doing considerably more useful work than the sparkling ornamental castle sitting next to it.
Common cycling mistakes
“I left the aquarium running for a week”
Running a filter moves water.
It does not, by itself, prove that the biological filter can process ammonia.
You need an ammonia source and you need to test what happens to it.
“I added bottled bacteria, so the tank is instantly cycled”
A bacterial starter may help establish the biofilter.
It doesn't remove the need to test.
Use the product as instructed and verify that the system is actually processing ammonia and nitrite before relying on it to support livestock.
“The water is crystal clear”
Water clarity tells you very little about ammonia and nitrite.
An aquarium can look beautifully clean while its water chemistry is anything but healthy.
Test rather than guess.
“I'll just add one hardy fish”
Hardiness should not be used as permission to expose an animal unnecessarily to poor water quality.
If the aquarium is empty, fishless cycling gives you another option.
Use it.
“I replaced the filter media because it was dirty”
Be careful.
Established biological media is one of the main places your nitrifying community lives.
If reusable media becomes clogged, gently cleaning it is quite different from replacing everything at once.
Cycling doesn't end when the first fish arrive
This is worth understanding.
The nitrogen cycle isn't a project you complete and then forget about.
It is happening for as long as the aquarium is running.
When you add more fish, you increase the amount of waste entering the system. When you feed more heavily, the load increases. If a fish dies unnoticed, plants decay heavily or the biological filter is damaged, ammonia can rise again.
That is one reason I recommend stocking a new aquarium gradually rather than filling it to its intended population immediately.
Give the aquarium time to respond.
The best thing cycling teaches you is patience
Cycling an aquarium can feel frustrating because the tank looks ready long before it actually is.
I understand the temptation.
There is an empty aquarium sitting in your room. The local fish shop has exactly the fish you want. Surely adding them a few days early can't make much difference?
It can.
Those first few weeks are an opportunity to learn something that will serve you throughout the hobby: an aquarium runs according to biology, not according to our preferred timetable.
Test the water.
Watch the numbers change.
Let the biological filter establish itself properly.
Then add your fish to an aquarium that is prepared to support them.
Waiting isn't the most exciting part of setting up a fish tank.
It might, however, be one of the most important.