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Cycling an Aquarium: Fishless vs. Fish-in Cycle

Most of us have made the mistake of impulse buying fish from our local pet store.

When we get home, we fill our empty tank with dechlorinated water and immediately introduce our fish. In the following days, our new fish struggle to thrive. More often than not, the situation proves fatal and ends in losses.

Unfortunately, this is a common mistake amongst new fish owners and aquascaping hobbyists due to an abundance of misinformation.

medaka rice fish in aquarium

Many pet store employees don’t teach their customers proper fishkeeping husbandry. However, understanding the nitrogen cycle and knowing how to cycle a new fish tank will ensure your new fish thrive. It's important to cycle your planted tank before adding fish to a new setup.

Fish thrive in established ecosystems when beneficial bacteria and live aquatic plants are living in balance with one another. It’s helpful to understand the science behind what makes an aquarium healthy and sustainable for its inhabitants.

If you’re setting up a new planted aquarium, keep reading to learn about the two common ways to cycle a new tank — and which to avoid — so you can ensure a healthy and balanced environment for your fish. 

This article will go over:

What is the nitrogen cycle?

Why is cycling a new tank important?

What is a fishless cycle?

What is a fish-in cycle? (and why we don’t recommend it)

Final thoughts on cycling a new tank

nano aquascape

What is the nitrogen cycle?

Establishing an ecosystem within your aquarium is essential to keeping fish and other inhabitants thriving. The nitrogen cycle is the biological process that converts toxic compounds produced by your fish into less harmful compounds. 

The nitrogen cycle starts when your fish produce waste after eating. This waste then converts into a highly toxic compound called ammonia. At high enough concentrations, ammonia can kill your fish — unless there are beneficial bacteria colonies established in your tank.

In a cycled aquarium, beneficial bacteria consume ammonia and produce nitrites and nitrates, which are less harmful compounds to your fish. Also, live aquatic plants help absorb ammonia, which neutralizes toxicity, helps keep nitrite and nitrate levels down, and makes the aquarium a safer environment for fish.

guppy in aquarium

Why is cycling a new tank important?

Cycling involves adding beneficial bacteria and live plants to a new tank to increase its biological filtration capacity. The process prepares a new aquarium to handle the waste load of its future inhabitants. A cycled aquarium can efficiently convert ammonia into less harmful compounds, keeping your fish safe.

Cycling transforms a new tank from a sterile environment to a mature ecosystem in which microorganisms, live aquatic plants, and fish can grow and flourish. Cycling is essential to having an aquarium that is biologically healthy, visually stunning, and enduring over the long term.

Now that you understand the nitrogen cycle and why it is important, let’s go over two of the most common ways to cycle a new tank.

shrimp on seiryu stone aquascape

What is a fishless cycle?

Cycling a new tank without fish is our preferred method because it doesn’t put fish at risk of exposure to high concentrations of ammonia. The fishless method of cycling involves dosing fish food or other ammonia source into an empty tank, with the purpose of growing beneficial bacteria.

Beneficial bacteria then consume the ammonia produced by the decaying fish food or other ammonia source. After consuming toxic ammonia, the bacteria produce nitrites and nitrates as a byproduct. The conversion of ammonia into nitrates can be tracked daily using an aquarium water testing kit.

This method works but takes a very long time to establish enough biological filtration to support the nitrogen cycle. You can expect it to take anywhere from a few weeks to a few months to fully cycle a new tank with the “ghost feeding” method. 

To speed things up, you can add used filter media from an established tank to your new tank’s filter. This encourages beneficial bacteria to quickly colonize your filter and cycle your tank in a matter of weeks. Only use media from a trusted source to minimize the risk of introducing unwanted pests and pathogens to your new tank.

To further accelerate cycling times, you can purchase bacterial additive products like SL-Aqua Black More which contain live, nitrifying bacteria cultures. These products are added during the cycling process to support existing bacterial colonies in your aquarium. As a result, they can reduce cycling time down to a few weeks.

red root floaters

What is a fish-in cycle? (and why we don’t recommend it)

This method involves adding a few hardy fish to an uncycled tank to induce an ammonia spike for bacteria to feed on. Performing frequent water changes on a regular basis is necessary with this method to keep ammonia levels within a tolerable range for fish. Like a fishless cycle, you can track the conversion of ammonia into nitrates with an aquarium water testing kit.

We don’t recommend this method for a few reasons. The main concern with fish-in cycling is that it's extremely stressful on the fish when they're introduced to an environment that can't effectively process ammonia. An uncycled tank simply lacks the biological filtration (beneficial bacterial colonies and live aquatic plants) required to effectively turn ammonia into less toxic nitrites, and then into nitrates. This can potentially lead to spikes in ammonia concentration that increase the risk of disease and even death.

This method also requires frequent and regular water changes. You must consistently maintain your tank to reduce errors that lead to failure to complete the cycle. Frequent water changes can also cause big temperature and parameter fluctuations, which can stress your fish and result in losses.

This method should only be considered in emergency situations where you have fish that need a tank but no established tanks are available. To increase your chances of success, the addition of a bacterial starter temporarily detoxifies ammonia spikes in new setups. Bacterial additives also supplement the existing populations of beneficial bacteria in your aquarium.

As aquarists, our goal is to establish a healthy and balanced ecosystem for our aquatic pets. The last thing we want is to harm any living creatures in the process. We recommend the fishless method of cycling over the fish-in method because it's an easier, faster, and risk-free way of establishing a cycle in your new planted aquarium.

guppies swimming in aquarium

Final thoughts on cycling a new tank

While cycling a new tank takes patience, it’s a critical first step in ensuring a safe environment for the health and longevity of your fish. A properly cycled aquarium is safeguarded by its own biological filtration. This will make your regular aquarium maintenance easier while cutting down on losses and rates of disease.

A successful aquarium comes down to understanding it’s about creating an ecosystem. Just because a fish is alive doesn’t mean it’s thriving. Similarly, cycling is not only about watching colors change in test tubes. It’s about establishing a long-term environment that maintains itself and naturally fosters harmony across all inhabitants.

cherry barbs swimming in aquarium

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Joy M - December 11, 2023

i want to start an aquarium. I want to do the fish less cycling but i need more detailed instructions on the precise way to do this. I have a beautiful aquarium i inherited and would love set up a planted tank for a betta fish. i know he won’t need all 20 gallons but i would enjoy knowing he has plenty of room and i would get to use my stunning tank.

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