Diary Of A Fish-In Cycle: My Experiences With My New Tank!

Properly cycling a new aquarium is crucial, as it establishes the beneficial bacteria needed to maintain low ammonia and nitrite levels.

The two main methods for cycling an aquarium are the “fish-in cycle” and the “fishless cycle,” each with its own advantages and disadvantages.

I chose to use the fish-in cycle for my new 29-gallon (110-liter) community tank and wanted to share my experiences in this article.

Although the fish-in cycle is somewhat controversial in some circles, it can work well when done correctly, and its popularity is slowly increasing within the fish-keeping community.

What Is A Fish-In Cycle?

Nitrogen Cycle Cheat Cheet for beginners!
Nitrogen Cycle Cheat Sheet For Beginners!

A “fish-in cycle” involves cycling a new aquarium with a small number of fish from the very beginning.

The goal is to gradually increase the levels of ammonia, nitrite, and nitrate in the tank as you slowly add more fish over the coming weeks and months.

Your tank requires various beneficial bacteria colonies to create a safe environment for your fish.

There are two main types. The first type converts ammonia into nitrite, and the second converts nitrite into nitrate. Nitrate is usually removed through partial water changes or used as nutrients by live plants.

By gradually increasing the number of fish, you slowly raise the ammonia levels, allowing the beneficial bacteria time to develop healthy colonies that can manage the toxin levels.

Live plants are highly recommended when using a fish-in cycle. Fast-growing plants, such as Limnophila Sessiliflora, convert ammonia, nitrite, and nitrate into plant growth instead of leaving them as toxins.

In tanks without live plants, frequent partial water changes are necessary to maintain suitable water parameters until the beneficial bacteria colonies can handle the bioload.

There are several beneficial bacteria products on the market, such as FritzZyme 7, that may reduce the time required for a fish-in cycle. However, they are controversial, and I did not use them in my own tank.

Here are the main pros and cons of using a fish-in cycle in your tank.


  • Add Fish Early
  • Easy When Done Right
  • Natural Source Of Ammonia
  • Real-Time Monitoring


  • Potential Harm to Fish
  • Requires Regular Monitoring
  • Limited Initial Fish Choices
  • May Require Water Changes

How I Ran My Fish-In Cycle!

My Community Tank
My Community Tank

Here is the process I used for my fish-in cycle:

  1. Set up the aquarium and let it sit for 24 hours, allowing the heater to warm the water.
  2. On the second day, introduce the first group of fish and feed them sparingly throughout the first week.
  3. On the eighth day, evaluate the water conditions. If all is well, introduce the second group of fish.
  4. On the fifteenth day, reassess the water conditions. If satisfactory, add the third group of fish.

I added a cheap sponge filter to the tank during the initial setup and kept it running throughout the full cycle to rapidly build up the beneficial bacteria.

This method is a blend of the Father Fish method and the Aquarium Co-Op method which are both popular options when using a fish-in cycle and I tried to take the best aspects of both methods.

Water Parameters

My Tanks Water Parameters
My Tanks Water Parameters

I checked the water parameters of the tank each morning and used a large partial water change as a fail-safe if nitrates exceeded 50ppm, following the Aquarium Co-Op water change guide.

While some people check their parameters every three to seven days during a fish-in cycle, I highly recommend checking them daily if possible, since you have live fish in the tank.

I used an API Master Test Kit for my readings because it is easy to use and much more accurate than most water test strips.

If you are on a budget, you can use cheap water test strips, but I suggest using three or more strips for each reading and averaging the results, as they are not as accurate as a chemical test kit.

Building The Aquarium

My Aquarium After The Initial Setup
My Aquarium After The Initial Setup

I implemented a few key strategies when setting up the aquarium to ensure my fish-in cycle would proceed smoothly.

Live Plants

Adding live plants was the most crucial step. I chose the following:

  • Amazon Frogbit
  • 5 x Anubias
  • 3 x Java Fern
  • 2 x Bucephalandra Kedagang
  • 1 x Bolbitis Heudelotii
  • Java Moss

In hindsight, I realize I could have chosen better plants for my fish-in cycle, as most of these are slow-growing and only absorb small amounts of ammonia, nitrite, and nitrate from the water.

1 Week Of Amazon Frogbit Growth
1 Week Of Amazon Frogbit Growthg

The photographs above show the growth rate of the Amazon Frogbit in my tank during the fish-in cycle.

As you can see, it consistently grew rapidly throughout the process, absorbing significant amounts of ammonia, nitrite, and nitrate.

I believe the levels could have been kept even lower if I had included other fast-growing stem plants, such as:

  • Limnophila Sessiliflora
  • Hygrophila Polysperma
  • Ludwigia Repens
  • Rotala Rotundifolia

Use A Filter

Multiple Filtration Methods On The AQQA Sponge Filter
Sponge Filter In The Tank

I opted to add a cheap sponge filter to the tank for my fish-in cycle to help grow beneficial bacteria colonies as quickly as possible.

The two main types of beneficial bacteria needed to manage ammonia and nitrite levels in an aquarium thrive on oxygenated surfaces in the tank.

Unlike some other inexpensive sponge filters that only offer a sponge, the AQQA sponge filter includes a ceramic media bed in the base, which increases the available surface area for bacteria.

The surface agitation caused by the filter also helps to manage oxygen and carbon dioxide levels in the tank, so I highly recommend using one for your fish-in cycle.

While some people prefer to heavily plant their aquariums with fast-growing plants instead of using a filter, even Diana Walstad, who popularized the Walstad method, recommends using a small filter for water movement and surface agitation.

Deep Substrate

Using A Deep Substrate Layer
Using A Deep Substrate Layer

I also chose to add a deep substrate layer to the tank to maximize the surface area available for beneficial bacteria.

Many people mistakenly believe that beneficial bacteria live in the water of the tank, but this is not correct.

While ammonia, nitrite, and nitrate are present in the water, beneficial bacteria reside on the surfaces within the aquarium.

These bacteria thrive in heavily oxygenated water, such as that in and around the filter, but they can survive on any surface. Therefore, more substrate means more bacteria and fewer issues with ammonia and nitrite levels.

Additionally, deep substrates in planted tanks with root feeders offer the benefit of absorbing detritus and fish waste, converting it into nutrients for the plants instead of increasing toxin levels in the tank.

Adding Honey Gourami

Adding Honey Gourami To The Tank
Adding Honey Gourami To The Tank

After setting up the aquarium, I let the heater run overnight to gradually raise the water temperature to 77°F (25°C) before adding my first two fish.

The key to these initial fish is that they should be small, few in number, and hardy, with most people adding two to four fish at this stage.

I chose to add two honey gourami because they are relatively small, work well in a community tank, and are fish I personally enjoy.

It’s important to feed your initial fish sparingly during the first week. I added two granules of Bug Bites every other day to keep ammonia levels as low as possible.

Some people use hardy fish such as Danios for their fish-in cycles, but if you don’t want to keep Danios in your tank once it’s cycled, you then have to rehome them, which didn’t sit well with me.

Honey gourami, native to India, thrive in habitats flooded by monsoon rains each year, so they have evolved to tolerate fluctuating water parameters.

As one of my favorite fish, honey gourami were the perfect choice to start the cycle.

Adding Corydoras

Adding Corydoras To The Tank
Adding Corydoras To The Tank

After monitoring the water parameters daily and not seeing any significant spikes in ammonia, nitrite, or nitrate, I decided to add four corydoras to the tank on day 8.

I love the personality and high activity levels of corydoras, so they were always intended to be part of my community tank.

Besides their charm, corydoras serve a practical purpose by eating leftover food that falls to the substrate. I also target feed them to ensure they receive all the nutrients they need.

While corydoras are typically kept in groups of six or more, I added only four initially to avoid spiking the ammonia levels during the fish-in cycle. I can always add more fish later.

Adding Neon Tetras

Adding Neon Tetras
Adding Neon Tetras

The ammonia, nitrite, and nitrate levels remained within expected ranges with the corydoras in the tank, so on day 15, I added ten neon tetras.

Neon tetras are another popular community fish that look great, but some breeding lines can be weak and sensitive to water parameters.

I waited until the tank had been running for a few weeks before adding the neon tetras. By then, the beneficial bacteria levels were beginning to establish, which I hoped would help manage any potential issues when introducing the tetras.

As you’ll see in the next section, this was the correct approach, as there were no spikes in ammonia, nitrite, or nitrate after adding the neon tetras to the tank.

My Experiences With A Fish-In Cycle!

After tracking the metrics of my aquarium throughout the fish-in cycle process I thought it may be helpful to share the various data points with my readers.

Although each aquarium is different, my data can serve as a benchmark for your own tank to help you get a rough idea of what to expect.

My Stocking Levels During The Fish-In Cycle
My Stocking Levels During The Fish-In Cycle

First up, we have the stocking levels of the tank and as you can see, there is a steady upwards trend of the number of fish in the tank with more fish being added each week.

Unfortunately, I did lose one corydoras on day eleven but I am confident that this is due to the fish having red blotch disease rather than it being due to problems with the fish in cycle process.

As you can see from my graph above, that corydoras was the only fish I lost during my fish-in cycle, and at the time of writing, that is still true.

Ammonia Levels

Ammonia Levels During The Fish-In Cycle
Ammonia Levels During The Fish-In Cycle

The ammonia levels of the tank did jump up and down throughout the fish-in cycle but this is totally normal when cycling a new tank, especially one with so few fast-growing plants.

As you can see in my graph, ammonia levels peaked at 0.25ppm multiple times with this being relatively safe for healthy fish for short periods of time.

The ammonia levels fell after each 0.25ppm spike giving me confidence that the beneficial bacteria and plants in the tank were doing their job.

I just want to quickly say that an ammonia level of 0ppm is ideal but this is usually only achievable in an established tank that has thriving beneficial bacteria colonies.

Maintaining a consistent 0 ppm ammonia level in a new tank is extremely difficult and unlikely even if you load your filter up with established filter media from one of your other tanks.

Nitrite Levels

Nitrite Levels During The Fish-In Cycle
Nitrite Levels During The Fish-In Cycle

I was actually surprised by the nitrite levels in my tank during the fish-in cycle as there was an initial spike up to 0.375ppm but the nitrite levels quickly stabilized at 0ppm.

This was not expected and I actually thought that there may be a problem with my nitrite test bottle in my kit so I checked the tank with test strips that also came back with a 0ppm reading.

I would guess that this is due to nitrite being the second toxin in the nitrogen cycle and the beneficial bacteria and live plants using it up as quickly as possible.

The live plants will also be using large amounts of the ammonia in the tank too so not all of the ammonia from the previous section will be converted into nitrites either.

Nitrate Levels

Nitrate Levels During The Fish-In Cycle
Nitrate Levels During The Fish-In Cycle

The nitrate levels in the tank hit a high of 7.5 ppm but they were usually far lower than this throughout the cycling process.

Most people recommend that you keep your nitrate levels below 10 ppm as a safety precaution but most fish will be fine with nitrate levels as high as 20 ppm.

If you remember, earlier in the article I mentioned that I had a fail-safe of doing a partial water change in the tank if the nitrate levels got as high as 50ppm but thankfully, we never even came close to that.

Nitrate levels are a little different than ammonia and nitrite levels as there is no beneficial bacteria that will convert them into other substances.

This means that you usually have to manually remove nitrate with a partial water change or add live plants that will absorb the nitrate as a nutrient.

I didn’t see the need to do a partial water change during the fish-in cycle so the lowering of the nitrate levels must be due to the live plants.

How Do I Know My Fish-In Cycle Is Complete?

The cycle is considered “complete” once you are able to feed your fish normal amounts of food for a week, and ammonia and nitrite levels stay at 0 ppm

Aquarium Co-Op

Most people consider their fish-in cycle to be complete once the beneficial bacteria levels are large enough to consistently keep the ammonia and nitrite levels at 0 ppm.

Introducing new sources of bio-load, like adding fish, will result in an adaptation phase where ammonia and nitrite levels may spike as the beneficial bacteria in the tank need to adjust to the increased bioload.

Should I Do Water Changes During A Fish-In Cycle?

My Water Siphon In My Tank
My Water Siphon In My Tank

Water changes may not be essential when doing a fish-in cycle, especially if you have live plants in your aquarium.

The main purpose of a water change during a fish-in cycle is to remove water that is high in ammonia, nitrite, and nitrate and replace it with fresh, clean water.

If you slowly ramp up the stocking levels of your tank to let your beneficial bacteria colonies grow as you add more fish and you have live plants using these toxins as nutrients the chances of your tank needing water changes are small.

Some people do still like to do water changes during a fish in cycle but I would recommend against doing them just for the sake of doing a water change.

Adding fresh water to a tank can cause problems with temperature shock, pH shock, and chlorine.

If you accidentally forget to use a tap water conditioner on the fresh water then the chlorine and chloramines can destroy your beneficial bacteria colonies.

In my opinion, a partial water change should only be carried out if your water test kit reveals high levels of toxins in your aquarium as per the following levels.

Ammonia0.5 ppm
Nitrite0.75 ppm
Nitrate50 ppm

Final Thoughts

The “fish-in cycle” method, while often debated, can be successfully executed with diligent monitoring of water parameters and careful selection of fish species.

My experiences confirms the importance of gradual additions to the tank and understanding the natural habitats and requirements of the introduced species.

With regular checks on ammonia, nitrite, and nitrate levels, this approach can offer a safe and effective means to set up a new aquarium.

As with all aquatic endeavors, thorough research and patience are key to ensuring the health and well-being of the tank’s inhabitants.