What is Fish Tank Cycling—How Do You Cycle a Fish Tank

By Eddie Waithaka @aquariawise

What is Fish Tank Cycling—How Do You Cycle a Fish Tank

When setting up a new tank, cycling is a critical event that you’ll want to understand. But for a new fish keeper, the idea and reason for cycling can be at best a crafty.

So, what is fish tank cycling really?

In the simplest terms possible, fish tank cycling is a four-stage process with the sole purpose being to establish a colony of healthy bacteria that clean the water of harmful chemicals caused by fish poop, leftover food, rotting plant debris, and any other organic waste that rise ammonia level in an aquarium.

In a nutshell: You feed your fish, your fish produces waste (ammonia), and the bacteria absorb the toxic waste so that your water remains safe for your fish to live in. In other words, the process sets up an ecosystem that can process ammonia without killing your aquarium inhabitants.

The four-stage process is also called the nitrogen cycle, and your filter, substrate, and decorations are the target hosts for the beneficial bacteria, though the filter media is the main-housing-unit of the three.

What are the main target elements for the healthy bacteria, you ask!

Majorly, the bacteria colony break down ammonia created by fish waste and organic debris into nitrites, then to nitrates, which are barely harmful to tropical fish especially in lesser concentrations.

And, what will happen if you add your fish in a tank that is not cycled?

Adding your fish in a tank that is not cycled will result in ammonia poisoning with the chemical elements eating away your fish’s scales, gills, and skin, then advance to compromise the fish’s internal organs such as the liver, brain, and the central nervous system.

The on-set sign of ammonia poisoning in fish is gasping at the surface of the water for air, then the gills will take on a red or lilac color, making them look like they’re bleeding.

This is also an easy (albeit harsh) way to tell if your aquarium is well cycled or if there is an ammonia spike such as when a filter fails, a tank is overstocked, or when you are overfeeding your fish.

Against that background, please read on for more insight on how to cycle a new fish tank, and prepare a healthy, thriving environment for your fish to live in.

How Long Does It Take For a Fish Tank to Cycle?

On average, the fish tank cycling process takes between six (6) and eight (8) weeks, but there is a caveat!

If you trigger the process by introducing bacteria colonies using a filter media from an established tank or by adding something like Safe Start, the process could take little to no time.

But it will take longer (the said 6 to 8 weeks) if you plan on cycling your aquarium by adding fish slowly and letting it stabilize between adds. Adding ammonia manually to jumpstart the process will also take up to a month for the nitrogen cycle to complete.

Comparatively, freshwater aquariums are usually more forgiving than saltwater tanks and can take as little as four (4) weeks (one month) for the whole process to complete.

These setups can also hold several small, hardy fishes like mollies, barbs, and danios during the nitrogen cycle, especially if you plan on keeping these fishes for the long haul.

I’ll talk more on that a little later though!

For now, let’s look at what happens during the six (6) to eight (8) week while your fish tank cycle.

Nitrogen Cycle Aquarium Timeline

As I mentioned before, the whole fish tank cycling process (Nitrogen cycle) happens in four main stages.

Below is a brief scan of the process before I take you through the finer details of the cycle.

  1. Introduction of Ammonia
  2. Nitrosomonas, Gram-negative, rod-shaped bacteria convert ammonia to nitrites
  3. Nitrobacter, Gram-negative, rod-shaped bacteria convert nitrites to nitrates
  4. Plants use nitrates and ammonium as fertilizer. The nitrates are also expelled during the weekly water changes

With that out of the way, let me take you through the five (5) stages in more detail.

Stage #1 — Nitrogen Introduction in Fish Tank

To start the Nitrogen cycle, the first thing you want to do is introduce a light stock of hardy fish into your aquarium, but make sure you have a filter in the tank to begin with.

Of course, adding the fish is only right if you decide not to go the fishless cycle route.

You then want to feed your fish for them to release their waste into the water column. Any uneaten fish foods will also add to the waste load in the tank, which then triggers an ammonia spike.

As you may already know, all decomposing materials like fish poop and pee, dead fish or plants, and leftover food breakdown and release ammonia which is pretty virulent to fish and need to be converted into something less toxic.

Essentially, Ammonia is a compound of Nitrogen, characterized by a pungent smell as evident in fish tanks that are poorly maintained.

Therefore, stage two (2) is figuring out how the healthy bacteria help reduce the harmful ammonia.

Please note that some aquarist choose to manually add pure ammonia in their fish tanks to trigger the Nitrogen cycle, as opposed to adding and feeding fish in the tank for their waste load.

Stage #2 — Bacteria Convert Ammonia to Nitrites

Once the ammonia levels in your aquarium levels start to rise, which usually happens from the third day after introducing fish, nitrite forming, Nitrosomonas bacteria will develop and convert the ammonia to nitrite.

Nitrosomonas is a genus of gram-negative, rod-shaped bacteria that oxidize ammonia into nitrites through a metabolic process known as nitritation.

Consequently, ammonia levels in your tank will decrease, as the nitrite levels increase. Mostly, the nitrite levels start to raise anywhere from the end of the first week to the beginning of the second week after you introduce the fish or ammonia.

Stage #3 — Bacteria Convert Nitrites to Nitrates

The same way nitrite forming bacteria develop when ammonia levels start to increase, Nitrobacter, which are nitrate forming bacteria will establish when the nitrite levels start to come up.

Nictrobacters are another genus of rod-shaped, gram-negative bacteria, but instead of oxidizing ammonia, they act on nitrites turning them to nitrates, which are not harmful to aquarium fish, albeit in large concentration.

Anytime from the second week of introducing your fish, the Nitrobacters will develop and level the nitrites at zero in the following days or week.

Stage #3 — Plants Use Nitrates As Fertilizer

Although nitrates are not harmful, large amounts can gradually cause health problems in fish. So, you want to make sure the levels stay below 20ppm.

And that’s where plants and water changes come in.

Aquariums plants use up nitrates from the water column, in the same way, plants that grow on land benefit from nitrates in the soil. But when you have zero live plants in your aquarium, regular water changes are recommended to help reduce nitrates.

Usually, ten (10) percent weekly water changes are acceptable.

And ultimately, make sure that ammonia and nitrites levels are at 0ppm and nitrates at 20ppm before adding fish in your newly cycled fish tank.

Can You Cycle Your Tank in 24 Hours?

Before I share my thoughts on whether it’s possible to cycle an aquarium in 24 hours, please take a second and give the theory another thought.

How possible is it to hasten a natural process that takes more than a month, sometimes two, and bundle it up into a mere 24 hours.

Im sure you’ve seen products in the market that are said to do exactly that, but I think that is nothing more than a marketing gimmick meant to trap new, unsuspecting fish keepers.

So, I will say with no hint of irony whatsoever, it is not even remotely possible to fully cycle a fish tank in 24 hours. Yes, you may be able to jumpstart the process and have some level of success keeping a few hardy fishes is there, but the nitrogen cycle will still take its due course.

Even so, there is a chance you might be able to hasten the cycle to a reasonably short duration, to maybe three weeks or so.

How to Cycle a Fish Tank Fast (“instantly”)

Away from using store-bought products such as Tetra safestart to instantly cycle your fish tank, I would suggest using old filter media that has plenty of bacteria colonies already established.

Add as many as four (4) old sponges loaded with beneficial bacteria in your new tank filters, along with several sacks of gravel from an established aquarium.

A few decorations from the mature fish tank will also help because they host bacteria colonies as well.

In case you do not have a second aquarium you can steal the sponge and gravel from, ask a friend or any amiable fish keeper in your local area since they regularly change their filter media; and they tose them anyway.

Be that as it may, you can also use store-bought nitrogen cycle accelerators, which work too, but not certainly. From my experience, you are likely to have better success with Biospira and Tetra SafeStart, but I cannot vouch for Dr. Tims.

Not that the former does not work, I just haven’t heard of anyone who used the product successfully, not even in forums, and I’m in many of those.

One last thing I recommend if you really need to cycle your tank fast is store-bought live nitrifying bacteria. Use the trick especially if you can’t get old filter media for use in your fish tank.

Can You Cycle a Tank Without Fish

Yes, you can cycle a fish tank without fish. In fact, it is recommended to avoid new tank syndrome, which refers to the effects of fish tank cycling on fish.

However, some experts argue that the best way to avoid new tank syndrome is to properly cycle your aquarium with a small number of fish and keeping up with your water changes.

Still the fact still remains, you can cycle a tank without the fish.

So how do you cycle a tank without fish?

The short version of it is by adding pure ammonia in your new tank.

How to Cycle a Tank Without Fish

To start the fishless cycle, set up your new tank making sure all the equipment including the filter is functioning and your water is dechlorinated, before adding ammonia to trigger the nitrogen cycle.

Use five drops of ammonia per 10 gallons of water daily until the concentration rise to five (5) ppm or slightly higher. Once the nitrites start to rise, reduce the ammonia input to three (3) drops per day.

Only stop adding the ammonia when the nitrite levels reach 0ppm and stay there.

Its also perfectly OK to add a lot of ammonia at once and let the tank sit, and only begin to add more when the levels begin to drop.

That said, do not wait for the ammonia levels to get to 0ppm to add your fish. Instead, add them when the ammonia tests are quickly dropping over a day, and nitrites levels have risen and then dropped back to 0ppm.

And before you add the fish, do a 50 percent water change to expel excess nitrates in the water and any excess ammonia. Then add your fish in the tank within the next few hours to start producing waste to keep the biological filter healthy.

I should also point out that the ammonia you use need be pure with no added colors, perfume, detergents or anything else that can potentially harm your fish.

One last useful hack when cycling your fish tank with pure ammonia without fish is to do continuous testing because it’s the only way to know if there is enough ammonia in your tank.

How to Cycle a Tank with Fish (Goldfish?)

Rest this title be a little misleading, please note that goldfish are not ideal for cycling a fish tank as they are quite vulnerable to disease. The only reason I have ‘goldfish’ on the heading because this is a common concern regarding cycling aquariums with fish.

Now let’s get into cycling with fish in the tank.

For starters, fish-in cycling is the most common approach used by both beginners and experienced fishkeepers alike. Basically, the logic is the same as in fish-less cycling, but instead of using pure ammonia to start the nitrogen cycle, here the waste produced by the fish you add is the trigger.

As I’ve mentioned (more than once I must say), fish poop, pee, and leftover food decompose and release ammonia in the aquarium, then beneficial bacteria convert that waste into less harmful nitrates.

To start the fish-in cycling process, set up the essential equipment in your tank, then only add a few hardy fishes. I recommend adding one small fish for every 10 gallons of aquarium water.

After that, start feeding the fish lightly and gradually increase the amount over the span of time it’ll take your fish tank to cycle.

The one thing you do not want while cycling your tank is the fish producing to much waste with not enough bacteria to breakdown the ammonia, so try not to overstock your tank or overfeed your fish.

Lastly, test your water frequently for ammonia and nitrites, followed by partial water changes anytime the levels go above 0.2 ppm.

What Fish Are Good For Cycling a Tank?

As you would expect, not all tropical fish kept in aquariums are good for fish tank cycling. Only a few species are hardy enough to survive in the harsh conditions inside a fish tank that’s not fully cycled.

Also, small fishes and a few medium-sized species are best for such use as opposed to large more susceptible fish types like cichlids and discus.

Below is a list of good fish (in my opinion) for cycling a new fish tank.

  1. Guppies
  2. Zebra Danios
  3. Tiger barbs
  4. Mollies
  5. White cloud mountain minnow
  6. Cherry barbs

Again, hold off on goldfish; they are quite vulnerable to diseases.

That’s all for this post.

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