How to Cycle a Tank: the Dark Start Method

How to Cycle a Tank with the Dark Start Method

If you have been in this hobby for a bit you will probably know that there are many ways to cycle our tanks. It can be the Dry Start Method, the Fish-In Cycle, the Fishless cycle, and the Cycle with plants. Nonetheless, the Dark start method is the new guy in the town and it is rapidly gaining popularity.

The Dark start method can be a great alternative to all other ways, as it simplifies the process and reduces the chance of error. Following this method, we can cycle the tank with no light and no water changes. In addition, the risk of developing algae is minimal.

This article will dive into why and how to cycle the tank with the dark start method along with the pros and cons to keep in mind when considering if this option is right for you.

Quick Comparison between the Dark start method, the Fishless cycle, and the Cycle with plants 

  Fishless cycle Cycle with plants Dark start method
Ammonia source Mostly bottled ammonia Soil/bottled ammonia Mostly the soil
Lights on/off On On Off
CO2 Off On Off
Fertilizers No Yes No
Water changes Lots Some No
(during cycling)
Yes/no Yes (many) No
(during cycling)
No No No
Plant Melting (risk) High Medium Low
Algae bloom Yes (lots) Yes Very low/no
Cycling time Fastest Medium Longest

Basics: Why do We Cycle a Tank?

If you are new to this hobby, it can be confusing to know where to begin, what to look for and why we have to do some things.

To put it in a nutshell, the goal of the Nitrogen cycle is to build up beneficial bacteria in the tank.

These beneficial bacteria are used in the tank to break down harmful waste (ammonia (NH3)) into a less toxic form, first to nitrites (NO2) and then down to nitrates (NO3).

Ammonia nitrite nitrate formula

Once this cycle is done your tank should not have ammonia and nitrites. Any non-zero reading in a cycled tank is a huge warning sign and should be dealt with immediately!

Cycling tank means that we are establishing beneficial bacterial colonies to make the environment safe for the livestock.

The nitrogen cycle is THE MOST important process in the aquarium.

If we skip it there is a very high chance that your pets (fish, shrimp, crayfish, etc.) will quickly poison themselves.

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Dark Start Method: a Step-by-Step Guide

  1. Add active substrate and hardscape into the tank.
    Optional: add bottled beneficial bacteria.
  1. DO NOT add any plants or livestock.
  2. Fill a tank with water.
  3. Turn off lighting.
  4. Turn on filtration.
  5. Turn on aeration.
  6. Let the tank cycle for 4-8 weeks.
  7. Do a massive water change (up to 90%).
  8. Turn on lighting.
  9. Add plants and livestock.

Now let’s take a look at each step in detail.

1. Adding Active Substrate and Hardscape into the Tank

Substrate and hardscape (such as driftwood or rocks) not only play a decorative role, but also serve as part of your biological filter as it provides a viable medium for nitrifying bacteria to colonize, and harbors many micro fauna.

Every other surface you may have within your tank targets for bacterial growth. Although it is true that the majority of your bacteria are living in your filtration system, the hardscape and, especially, the substrate also contribute a lot to this matter.

There are two types of substrate:

  • Inert substrate – this is a substrate that does not alter water chemistry (PH).
  • Active (buffered) substrate – this is a substrate that leaches ammonia and, thus, alters water chemistry (pH).

Active (or buffered) substrates (for example, ADA Amazonia aqua soil, Mr. Aqua Aquarium Soil, Fluval Plant and Shrimp Stratum, etc.) will leach ammonia from a few weeks to possibly a couple of months. These ammonia spikes (and overall changes in water parameters) can be very dangerous even for plants.

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1.1. Why do we need an active substrate for the dark start method?

Even though beneficial bacteria are naturally occurring and will colonize the tank in due course, adding active substrate is a great way to catalyze the process.

The active substrate will leach ammonia which is needed to jumpstart the cycle. Basically, we use the active substrate as an ammonia source. This is not the same as simply washing the ammonia out of the substrate, which we can simply do in a bucket.

These bacteria will convert ammonia to nitrite which is converted to nitrate.

Note: To boost the process, we can also add bottled beneficial bacteria. However, this is optional.

1.2. Can we use an inert substrate for the dark start method?

Yes, we can. However, the main difference is that to start the nitrogen cycle we will have to introduce a bacterial source (ammonia) in the first place. After that, the process will be the same.

  • Add 1-2 drops of ammonia per 1 gallon (~4 liters) and no more.
  • There is no need to add ammonia until you get a reading of >2 ppm (as some people recommend). This is already a very high concentration!

2. DO NOT Add Any Plants or Livestock

Ammonia and nitrites are very toxic to all animals. These chemicals:

  • cause a lot of stress for our aquatic pets;
  • negatively affect their immune system, making them susceptible to parasites and other diseases including bacterial infections, fungal infections, or all types of different illnesses;
  • stun their growth rate;
  • cause gill damage;
  • destroy the excretory system, etc. 

As for the plants, many delicate plant species and tissue-cultured plants can melt during cycling. So, if you are planning to keep them in the tank, you need to think about it.

Thus, the main goal of the dark start method is to give time to allow for the ammonia and nitrite levels to decline. It will minimize plant melt or any risk for the livestock.

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3. Fill a Tank with Tap (or RO/DI) Water

The next step of the dark start method is to add water to the tank. So, now you need to decide what type of water you are going to use for that.

Depending on your tank setup, there are two options:

Tap water:

Generally, tap water is the most common source of water in freshwater aquariums. So, if you are going to run the tank on tap water eventually – use tap water right from the beginning.
Important: The main problem with tap water during cycling is that it contains impurities (phosphates, nitrates, silicates, iron, copper, etc.) that can be harmful and usually lead to outbreaks of nuisance algae.

RO/DI water:

It takes ordinary tap water and produces zero TDS. In other words, it is absolutely pure water without any elements in it. Because of it, you will have to remineralize RO/DI water with special additives later on.

So, if you plan on keeping sensitive animals (for example, Caridina or Sulawesi dwarf shrimp species) you will have to keep them in remineralized RO/DI water.

Important: The main problem with RO/DI (or distilled water) during cycling is that we are going to be doing a massive water change at the end of the cycling to remove the nitrates. That is why it will be just a waste of remineralizers.

So, should we cycle the tank with pure RO/DI water (without remineralizers)?

No, it is better to waste some remineralizers earlier to get better results later.

The point is that beneficial bacteria will not multiply as rapidly as they should in pure RO/DI water. These bacteria also need water with minerals. Therefore, cycling the tank with RO/DI water will be unnecessarily prolonged.

Therefore, I would still recommend going for remineralized RO/DI water (or at least tap water) during the dark start method.

What should you do if you dark cycled the tank with tap water but want to keep animals in RO/DI water?

  1. Do a large water change with remineralized water RO/DI water.
  2. Test your water parameters.
  3. Wait 1 week and do a 20-30% water change with remineralized RO/DI water.
  4. Wait 1 week and do a 20-30% water change with remineralized RO/DI water.
  5. Test your water. There should not be any changes in pH, KH, and GH.
  6. Add livestock.

4. Turn off Lighting

Algae are a diverse group of predominantly aquatic photosynthetic organisms. According to some studies, their photosynthetic efficiency is even higher than in plants.

It means that algae will thrive under low or high intensities.

However, depriving algae of light will ensure that they will not be able to grow and reproduce. In the absence of light, the algae population will not be able to photosynthesize further, hence resulting in a gradual die-off.

Due to the fact that there are no animals and plants in the tank, we are not limited in time. Hence is the name – “the dark start method”. Lights should be turned off until the tank is completely cycled.

Should we cover the tank? Will it be enough to just turn off the lights?

In my experience, covering the tank completely with black sheets is the best way because an ambient light that can get into the tank may cause a brown algae outbreak.

So, cover the tank and all sides.

However, if you keep the tank in a completely dark room with no lights – no need to wrap, just lights off, it will work for you as well.


  • Plants that are denied sufficient light for a long time (weeks) will eventually starve to death. That is why we do not add them at this stage.
  • Blackout is a very popular and extremely effective way to eliminate most types of algae without using any chemical or algaecide products.

5. Turn on Filtration

The aquarium filter is an indispensable component of almost any aquarium setup. It does a tremendous job at keeping the tank nitrogen cycle stable, therefore, making the environment safe, healthy, and habitable for aquatic life forms.

So, during the dark start method, leave the filter running all the time.

6. Turn on Aeration

The main reason why beneficial bacteria mostly reside in the filter media (up to 70-80%) is water flow. They need it to get easier access to oxygen and food.

Biological filtration requires lots of oxygen to grow and multiply.

Turn on your aeration and leave the filter running with the outlet rippling the surface, as this will help the colonies to grow by increasing the oxygen content of the water.

7. Let the Tank Cycle for 4-8 Weeks

Now you can sit back and be patient as the tank will cycle for a month or more by itself.

I repeat, the essence of cycling is to convert ammonia into nitrite and nitrite into less toxic nitrate.

Our goal is to get to 0 ammonia and 0 nitrites. Generally, in the dark start method, it can take from 4 to 8 weeks (depending on how much ammonia you had in the first place).

8. Do a Massive Water Change

That is probably the greatest advantage of the dark start method. NO NEED for any water changes during the cycling process. Just one large water change and we are good!

You will need the Test Kit to monitor the result of the cycling process. This is a must-have tool!

Without a test kit, it will not be possible to estimate the real condition of the growing nitrogen cycle (ammonia, nitrites, and nitrates).

Generally, at this point, ammonia and nitrite must be zero. However, you will have lots of nitrates. Do not panic, this is absolutely normal.

It is our job to remove nitrates via the large water change. Change as much as you can, it can be even up to 90% of your aquarium water.

Note: Do not worry, this will not remove the beneficial bacteria in your tank.

 If you need to remineralize your water, check out my article “How to Do and How Often to Do Water Change in Shrimp Aquarium”. The principle is the same except that you do not need to do it slowly (because we still do not have animals in the tank).

Tip: It is recommend planting before flooding the tank (see below).

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9. Turn on Lighting

For the dark start method, we need to complete the nitrogen cycle with lights.

  • All plants require light for photosynthesis.
  • Even nocturnal animals (such as shrimp, snails, crabs, crayfish, etc.) have shown that photoperiod plays an important role in regulating their reproductive cycle.

Lighting should be adapted to the needs of your animals and plants.

10. Add Plants and Livestock

Once the cycle is finished and the water parameters are met, technically, you can add plants and livestock.

However, if you want to play safe, it will be better to add only plants and wait at least 1 or 2 weeks before adding any animals. Let the tank establish and mature a little bit.

Tip: During water change, you can drain the tank as low as possible so you can plant easily. Only after that, it will be better to refill it with water.

Do not forget to check your water parameters daily.

You need to be absolutely sure that water parameters are stable and that there are no sudden ammonia spikes.

Important: Do not overstock and/or overfeed the tank after cycling or you can get a new tank syndrome.

The underlying cause of new tank syndrome is the lack of ample nitrifying bacteria to break down the waste produced by your aquarium pets alongside other sources.

This is especially important for animals that produce a lot of waste (like fish, freshwater crabs, crayfish, frogs, snails, etc.).

Add only a few animals, for a period of time until your tank is sufficiently populated with nitrifying organisms without the risk of causing a mini-cycle.

In Conclusion

Many aquarists have already tried the dark start method with great results. No light on, no algae, no plants, no fish, no need for daily changes – cycling the tank has never been easier and safer.

This method gives a much more stable start, especially for sensitive animals and plant species. This is important with substrates that contain and leach a high amount of ammonia.

The main downside though is that it extends setup time. However, our hobby is not cheap and we should never rush the process.

Pros and Cons of the Dark Start Method


  1. Time: This method requires more time compared to others. We also can’t control the cycling process. In addition, it will be better to wait another few weeks after planting before safely adding livestock.
  2. Aesthetics: We can’t décor your tank with plants right from the beginning. The covered tank doesn’t look great. Let’s be honest.


  1. Easy. Unlike fish-in cycling, fishless cycling, and cycling with plants, there is no need to add/or measure ammonia and nitrites all the time.
  2. No water changes: Just one large water change at the end of the cycle.
  3. Cheaper: No electricity costs, no fertilizers, etc.
  4. Maintenance: No algae or melting plants. If you have driftwood, it will water-log and lose/leach lots of tannin during the cycling.
  5. Safe: It will not harm your animals.
  6. Stable: Although the dark start method needs more time, it also means that, in the long run, beneficial bacteria will create an excellent balance in the tank.

4 thoughts on “How to Cycle a Tank: the Dark Start Method

  1. If using RO/distilled water, how much should we remineralise? The ideal TDS should be at?

    1. Hi Jo,
      The answer to your question will depend on what kind of aquatic pets you have in your aquarium (shrimp, crayfish, crabs, etc.), as the specific water parameters required for these pets will vary depending on the species.
      Best regards,

  2. hi, should the heater operate during the cycle?

    1. Hi Tony Cornish,
      If your room temperature is around 68-72°F (20-22°C), it will be sufficient.
      However, it is worth noting that the growth and reproduction of beneficial bacteria are temperature-dependent. According to studies, at a temperature of 65°F (18°C), the growth rate is reduced by 50%.
      Best regards,

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