Cycling a Tank for Aquaponics With or Without Fish

Illustration of the nitrogen cycle in aquaponics

Aquaponics sounds complex, but it’s really not…

It just takes a little bit of knowledge and you can be off to the races, growing plants and fish in perfect harmony.

In an aquaponic system, you attempt to create a symbiotic relationship between fish or other aquatic creatures, bacteria and hydroponically grown plants. The fish waste is a source of organic food for the plants and the plants purify the water for the fish. Bacteria are involved in converting the fish waste into less toxic substances that feed your plants.

But before we get to that state, we have to “cycle” the tank.

What Does Cycling Mean?

Illustration of the nitrogen cycle in aquaponics
Illustration of the nitrogen cycle in aquaponics. Source

Cycling is the process of introducing ammonia to the system to attract the bacteria required for successful aquaponics. Microbes are a critical component of an aquaponic system. The microbes are bacteria that convert ammonia created by the fish waste into nitrites and then nitrates. Plants use the nitrates, as it is a form of nitrogen they can pull into their root systems to promote growth.

You can cycle a tank that is to be used for aquaponics in one of two methods:

  • Using fish, or
  • The fishless method

Cycling using the fishless method will allow you to get your system up and running more quickly and will let you fully stock your fish tank in a couple of weeks.

Cycling using fish can be stressful to the fish and will require at least a month before your system is fully functional.

We will take a look at both methods and you can decide for yourself which method you will use for your aquaponic tank. Both methods employ testing of your water chemistry so you will need to have a good quality test kit that can measure PH, ammonia, nitrate and nitrite levels as well as a thermometer to monitor water temperature.

Method 1: Cycling Using Fish

Cycling aquaponics tank with fish

Using fish is the most natural way to cycle your aquaponic tank. The fact that you are using live animals complicates this cycling process as you need to take into account the health of the fish as the tank cycles. This is an added burden to the aqua gardener and should be taken into consideration when choosing a cycling method. You will need to keep a close watch on your water chemistry to protect the fish and be prepared to take corrective action quickly if necessary. In cases of very high ammonia concentrations you will need to be ready to change 1/3 of your tank’s water relatively quickly to keep your fish alive.

Ammonia Accumulation

Ammonia will begin to accumulate as soon as fish are introduced to the tank as a byproduct of their respiration and digestion. Without removing or converting the ammonia to a less toxic form of nitrogen it will reach dangerous levels. It can quickly rise to levels that will kill your fish so its concentration must be monitored and the levels kept low. This is best done by starting with only a few fish fed lightly once a day. Trying to fully stock your tank immediately will in all likelihood lead to tragic results for the fish. They will not be able to handle the ammonia spike likely to occur if too many fish are introduced at one time.

So now you have introduced a few fish to begin the cycling process. Perhaps you should use a few goldfish as you do not want to put any real prize specimens in your tank at this point. This is because during the cycling process you need to monitor your water chemistry daily and be prepared to make water changes if your levels are not correct. Fish that can handle these changes are best used and goldfish fit this category. You also should add some plants at the beginning of the cycling process so they can be ready to begin extracting the nitrates as soon as they are created.

Ammonia Conversion

In your tank, ammonia (NH3) is constantly converted to ammonium (NH4+). The reverse process also occurs. Ammonia is the form of nitrogen that is toxic to fish. Most standard test kits measure total ammonia and do not distinguish between the two types. Higher water temperatures and higher PH readings will cause more of the nitrogen in your tank to be in the toxic form of ammonia. This means you must also monitor your tank’s PH and temperature in addition to the ammonia levels. A balance needs to be found that will benefit plants and fish. Keeping a PH of 6.0 – 7.0 should serve this purpose during cycling and your mature aquaponic system should have a PH of between 6.8 and 7.0. Temperatures should be kept on the low side of what your fish can tolerate.

Caution needs to be taken when adjusting PH levels and you should always go slowly. Changing the PH too dramatically at once will harm your fish. If you find your PH is not correct do not modify it with chemicals by more than .2 per day.

Bacteria Growth = Good

You can speed up the growth of bacteria by using some gravel from a disease free and well established aquarium. This gravel will already be rich in the necessary bacteria and will give you a head start with your cycling. Other possible sources of bacteria useful to your tank are from filter media from an established tank. These materials can often be obtained at local fish stores.

Bacteria called nitrosomonas that will become attracted to your tank convert ammonia to nitrites. This substance is even more toxic to the fish but will quickly attract another bacteria called nitrospira which will turn the nitrites into nitrates which are harmless to your fish. Nitrates are also great food for your plants.

In daily testing of your water you are looking for the nitrite level to be below .5. Once your nitrites are at this level and you have detected nitrates in your water your tank is cycled and your aquaponics system is ready to be fully functional.

Method 2: Cycling Without Fish

Fishless cycling of aquaponics system

Cycling without fish has some advantages over using fish. One of the main benefits is that both the fish and the keeper will endure less stress while the cycling is proceeding. You do not have to be overly concerned with the water chemistry as there are no fish that can be negatively impacted by out of bounds PH or ammonia levels. You do not have to be concerned with controlling feeding to limit nitrite production.

Fishless cycling enables the procedure to be done more quickly because you can add ammonia in greater concentrations than if you were concerned with the health of your fish. You can expect your tank to be completely cycled in 10 days to 3 weeks going fishless where with fish it will take approximately 6 weeks. You also have more control over the amount of ammonia you are adding without worrying about impacting your fish. Though cycling with fish is the more traditional method you should consider fishless cycling for these reasons.

Adding Ammonia Manually

To cycle your aquaponic tank, using the fishless method you will need to add ammonia as there are no fish to produce it naturally. Ammonia can be obtained from a variety of sources. Pure ammonia is often available at your local hardware store. Be sure it is 100% pure ammonia and does not contain perfumes or other additives that will harm your system. You can also use crystalized ammonia which is available in some fishless cycle kits. This is more expensive than liquid ammonia but you will be assured of its purity and viability in your system. Human urine and decaying animal flesh have been used as ammonia sources but both have issues not found in the pure ammonia available. I’d go with either the liquid or crystal pure ammonia.

Once you have obtained your ammonia the cycling process is fairly straightforward. You can start by adding some plants to the environment. You then add ammonia to the tank. Using small increments and regular testing, stop adding ammonia when your test kit shows about 5 ppm. Take note of the amount of ammonia that you used and repeat this dosage daily. You are now testing for nitrites to be found at a level of 0.5 ppm.

After the nitrites begin to appear you know you have attracted the bacteria necessary for cycling. You should then lower the dose of ammonia, using only half of what was previously used. Continue dosing it daily until you find that nitrates are showing up in your water testing. Once their level gets to 5 to 10 ppm and your nitrites have dropped to zero you can add fish. Your tank is now cycled and your aquaponic adventure begins.

Which Tank Cycling Method to Choose?

When choosing a cycling method there are some factors to consider…

If you cannot easily obtain pure ammonia then cycling with fish may be the best alternative.

On the other hand if ammonia is easily available then fishless cycling will allow you to have your aquaponic system running more quickly and eliminate some stress that could have been generated for you and your fish.

Toby SandersAbout the Author

Toby Sanders has more than 15 years of experience in aquarium sector and the founder of Aquarist Guide. He enjoys sharing his aquarium knowledge to help people effectively build their own tank.