Cycling a New Tank

Cycling your aquarium is one of the most important steps in setting up a healthy, stable environment for your fish. It’s not the most glamorous part of the hobby, but skipping it can lead to frustration, constant water changes, and fish that don’t survive past the first few weeks. Whether you’re starting a freshwater or saltwater tank, understanding how to establish the nitrogen cycle is essential.

This guide covers everything you need to know about cycling an aquarium, including fishless cycling, fish-in cycling, and methods to skip the cycle altogether using mature media.

What is Aquarium Cycling?

Cycling refers to the process of establishing beneficial bacteria in your aquarium that convert toxic waste products like ammonia into less harmful compounds. These bacteria primarily grow on surfaces—especially in your filter—and turn ammonia into nitrite, then finally into nitrate.

This process can take several weeks, and while it may seem slow, it’s critical for your fish’s long-term health. Without cycling, toxic waste accumulates rapidly, stressing or killing your fish.

Why is it Important to Cycle Before Adding Fish?

Cycling gives your filter time to establish the bacteria needed to handle fish waste. Without this step, you’ll have to do daily water changes to keep toxins under control. Worse yet, you could end up dealing with dying fish, algae outbreaks, or even a full crash of your system.

A properly cycled tank is not only easier to maintain—it provides a much more stable and stress-free environment for your fish.

How to Cycle an Aquarium

Fishless Cycle

Fishless cycling is the safest and most effective method for setting up a new tank. You’re essentially feeding the bacteria with ammonia to simulate fish waste—without actually harming any fish in the process. You will need 3 simple things: A source of ammonia (fish food or chemical ammonia), a test kit and patience. 

1. Set up the tank completely, including the filter and heater.

2. Do not add fish.

3. Add ammonia to reach 2 ppm. You can use pure ammonia (make sure it contains no detergents—shake the bottle: if it foams, skip it) or a pinch of fish food that will decompose over time.

4. Monitor ammonia and nitrite levels daily. You’ll first see ammonia rise, then nitrite. This can take 1–2 weeks.

5. Wait for nitrites to spike, then drop to 0. This is the slowest part of the process and may take several weeks.

6. Check for nitrate buildup. When both ammonia and nitrites read 0 and nitrates are present, your cycle is nearing completion.

7. Do a 24-hour test. Dose ammonia back to 2 ppm and check after 24 hours. If both ammonia and nitrite are 0, your tank is cycled.

8. Add fish slowly. Introduce only one species or a few individuals at a time. Wait several days between additions to allow your bacterial colony to adjust to the increased bioload.

Pro Tip: You can speed up the process by seeding your tank with beneficial bacteria. The best source is filter media from a mature tank. If that's not available, clean soil from an area with plant life also contains nitrifying bacteria—just ensure it's pesticide- and fertilizer-free.

Fish-in Cycling 

Fish-in cycling should only be done in emergencies, like after a cycle crash, or if you already added fish before understanding cycling. This method is far more stressful for both you and your fish and requires constant monitoring.

Warning: Never attempt a fish-in cycle without a reliable test kit.

1. Set up the tank and add fish. Ideally, only one small fish in a large tank.

2. Test ammonia and nitrite levels at least once daily. Multiple times is better.

3. Perform large (50%) water changes anytime ammonia or nitrite exceeds 0.5 ppm.

4. Feed sparingly. Every other day is ideal until the cycle completes.

5. Wait for ammonia and nitrites to fall to 0. This can take up to 6 weeks.

6. Once complete, you can slowly begin adding more fish—one species at a time.

Most freshwater fish can tolerate brief exposure to 1 ppm of ammonia or nitrite, but it’s a gamble. Saltwater fish, on the other hand, are far more sensitive due to the higher toxicity of ammonia at high pH. Fish-in cycling is not recommended in marine tanks unless you have well-cured live rock.

No Cycling Cycling

This method involves using enough mature filter media from an established tank to bypass the cycling process entirely. If your new tank's filter is filled with well-established bio-media, the bacteria are already present in large enough numbers to process waste immediately. Saltwater tanks with sufficient live rock and flow can also use this approach effectively.

1. Add mature media directly to your filter.

2. Test it: Add 2 ppm ammonia and check levels after 24 hours. If ammonia and nitrite are still 0, your tank is ready.

3. If fish are already present, monitor ammonia and nitrite closely for 24–48 hours to ensure the system is working.

This is the fastest and most reliable method, but it depends on having access to mature media—which isn’t always possible for new hobbyists.

Final Thoughts

Cycling your tank is the foundation of a successful aquarium. Whether you’re taking the traditional fishless route, managing an emergency fish-in cycle, or seeding with media from another tank, the end goal is the same: to build a thriving colony of beneficial bacteria that can process fish waste safely.

If you're patient and plan ahead, cycling doesn’t have to be difficult—and your fish will thank you with long, healthy lives.