"Aquarium Cycling" is the process of growing beneficial bacteria that turn toxic fish waste into safer compounds. This invisible ecosystem, known as biological filtration, is what keeps your tank alive. Without it, ammonia and other toxins rise rapidly, and fish are forced to live (and often die) in their own waste.
All the living creatures in your aquarium produce waste in the form of ammonia. Any leftover fish food or dead and decaying plants or fish will also increase ammonia levels in your tank. Ammonia is highly toxic to fish and other aquatic life
In simple terms, the nitrogen cycle is the process by which ammonia (toxic!) is converted to nitrite (toxic!) and then to nitrate (safer). Each step in the cycle requires specialized bacteria to convert the compounds. These nitrifying bacteria, like Nitrosomonas, Nitrospira, & Nitrobacter, are beneficial bacteria in aquarium systems and are essential for a healthy, thriving tank.
Due to their toxic nature, ammonia and nitrate levels in a healthy aquarium should always be zero.
Nitrate, on the other hand, isn’t as harmful, and can even be beneficial if you are growing aquarium plants. Ideally, you’d want to keep your nitrates under 20 ppm, so you will need to perform a partial water change any time the levels get too high. If you have a densely planted aquarium, you likely will have low nitrate levels and may even need to add some back in in the form of plant fertilizers! This is one of many reasons why I personally add plants to all of my aquariums.
Now that we’ve covered what tank cycling is, let’s dive into how to get it started!
Before you embark on your tank cycling journey, be sure to gather a few supplies:
Although some will grow on your hardscape and substrate, the vast majority of your beneficial bacteria live in your aquarium filter where they have access to flowing, oxygen-rich water. You can use any of the following to cycle your tank:
If you're not sure which filter type is right for you, check out this video on Finding The Best Filter For Your Setup!
Biological media is the material inside your aquarium filter that provides a place for beneficial bacteria to grow. It is typically made from highly porous materials designed to maximize surface area for growth. Common examples include:
All of these materials are designed with tiny pores and textures where bacteria can attach and form stable colonies. If your aquarium filter didn't include any media, be sure to pick up a few sponges or bio balls before setting it up.
Note: when it comes time to clean your aquarium filter, be sure to only rinse your bio media and sponges in tank water. Do NOT rinse under the sink and do NOT replace your bio media entirely or you will cause a cycle crash.
Tap water often contains chlorine or chloramine to kill off unwanted bacteria and other microorganisms. Unfortunately, this includes the beneficial bacteria you're trying to grow in your aquarium. Use a dechlorinating water conditioner or reverse osmosis water when setting up and refilling your tank to keep your biological filter safe. Popular examples include:
Building your bacterial colony will require a steady source of ammonia to feed on. For fishless cycling, you can use any of the following:
You will need to test your aquarium water on a regular basis during the cycling process. The main parameters to watch are ammonia, nitrite, and nitrate, but knowing your pH is helpful when troubleshooting. Popular choices include:
I like test strips or the Seachem Ammonia Alert for the very early stages of cycling when you're just waiting for something to happen, but as the cycle progresses you will be better served by a titration (dropper) based kit for the most accurate and reliable results.
(Optional, but recommended)
Botted bacteria options contain live or dormant nitrifying bacteria to help kickstart your cycle. Although they are helpful, they are not magic-- you cannot pour bottled bacteria in and add fish the same or next day, no matter what the clever marketing says. The beneficial bacteria will populate your tank whether you use a bottled bacteria or not, but I've personally found bacteria starters to be helpful for speeding up the process. A few popular options:
(Optional, but recommended)
Bacteria thrives in warm conditions, and having too cold water can stall or slow your cycle. If you live in a colder climate or keep your house below ~75 degrees, you'll likely need a heater for cycling.
Keep in mind most aquarium fish species are tropical, so you'll need a heater regardless once you're ready to add fish.
Most importantly, you will need patience. The cycling process can take anywhere from 2 - 6 weeks (or more!) and requires planning and preparation. Don’t expect to go to the fish store and bring home a new tank and fish on the same day. Similarly, don’t spend a fortune on expensive, delicate live plants that will melt away in your uncycled tanks. Keep it slow & simple for success.
There are two main methods hobbyists use to cycle a tank: fish-in cycling and fishless cycling.
For ethical reasons, we recommend a fishless cycle.
Set up your tank, filter (with biological media inside), heater, substrate, and any hardscaping decor or hardy live plants.
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Fill up your tank slowly with dechlorinated tap water or Reverse Osmosis water. Aim the water stream towards a large piece of hardscape like rock or wood, or use a plate or plastic container to avoid kicking up ecessive substrate while filling.
Pour starter bacteria (if using) into your filter, ideally directly onto the biological filter media. You can also pour a bit into the main tank around the decor, hardscape, substrate, & plants. If you skip this step, the beneficial bacteria will still show up in your tank eventually, but it may take longer.
Add an ammonia source to feed the bacteria. This can be pre-bottled ammonia, fish food, or a bit of frozen shrimp or fish from the grocery store.
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Test for ammonia daily or every other day using a test kit. The goal is ~5ppm. Continue adding ammonia source daily or per bottle instructions until ammonia levels reach this level.
Once you reach ~5ppm ammonia, test for both ammonia and nitrite over the next few days. Keep testing until you see ammonia levels drop and nitrite levels begin to rise.
Once you see a spike in nitrite, you're halfway there! Now it's time for the second type of bacteria in your tank to get established. Keep testing all three parameters (ammonia, nitrite & nitrate) over the next few days. Aim to keep ammonia levels around ~1-2ppm. In a few days, you should see nitrites drop and nitrates rise.
Once your nitrates spike, add more ammonia and watch ammonia go from → nitrite → nitrate over the next day or two.
Repeat steps 6-8 until ammonia & nitrite both hit zero within 24 hours. Congratulations! You’ve now established an aquarium cycle!
Add your first tank inhabitants. Don't go overboard! Stick to a few small fish, handful of shrimp, or a large snail. You can add more slowly over time to give your biological filter time to catch up. If you have to wait longer before adding fish to your aquarium (shipping times, stock issues, still undecided on stocking, etc), be sure to keep your bacteria fed with supplemental ammonia in the meantime.
There are a few things that can slow down or even halt your cycling process entirely. For best results, avoid the following:
Your tank is ready when:
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Ammonia = 0 ppm Nitrite = 0 ppm Nitrate = 5 - 40 ppm |
And the results are stable for several days.
Cycling your first aquarium is a practice in patience and expectation management, but it gets much easier the more you do it. In fact, cycling your second aquarium can be done in a fraction of the time by using filter media or sponge filters from your already-established tank!
If you run into issues and need help troubleshooting, feel free to reach out to our aquarium experts at (763) 432-9691. Happy fishkeeping!
Abbey has been a proud member of the BRS team since 2020. With a background in teaching, she’s passionate about educating new aquarists and making the hobby approachable. Her true love is freshwater planted tanks—especially those filled with shrimp, nano fish, and lush, nature-inspired aquascapes. Abbey enjoys finding creative, practical solutions to common aquarium challenges and sharing tools, tips, and inspiration through her writing.
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