Published on November 2, 2025
Cycling your aquarium for the first time is one of those rites of passage in the aquarium hobby that everyone must do. When people first come into contact with the subject, it’s confusing and intimidating because it’s often referred to as the nitrogen cycle. Say what now?? I just want my dang Betta fish to live. I didn’t realize I was going to have to learn chemistry!
This is not a definitive guide to cycling an aquarium. This is how I cycle aquariums. I use a fishless cycle by dosing ammonia but there are other methods which are not covered here because I do not use those methods. This is the best method in my estimation.
What is …
Published on November 2, 2025
Cycling your aquarium for the first time is one of those rites of passage in the aquarium hobby that everyone must do. When people first come into contact with the subject, it’s confusing and intimidating because it’s often referred to as the nitrogen cycle. Say what now?? I just want my dang Betta fish to live. I didn’t realize I was going to have to learn chemistry!
This is not a definitive guide to cycling an aquarium. This is how I cycle aquariums. I use a fishless cycle by dosing ammonia but there are other methods which are not covered here because I do not use those methods. This is the best method in my estimation.
What is the nitrogen cycle?
There are many great explanations on the internet, but in short, the nitrogen cycle is the process by which bacteria convert ammonia to nitrites and then nitrites to nitrates, making your aquarium water safe for fish.
This cycle is important because fish waste contains ammonia, which is toxic for fish and other aquatic critters.
“Cycling” your tank means purposefully growing the bacteria colonies (beneficial bacteria) needed for the nitrogen cycle to happen in your aquarium.
Prerequisites
In addition to your aquarium and the equipment to make it work (lights, filter, heater), you’ll want a few additional items before starting the cycle.
- Water conditioner: I use API Tap Water Conditioner, but there are many different brands. Dave over at Aquarium Science says to use conditioners that say “Sodium Thiosulfate” on the bottle in the ingredients list. Many people swear by Seachem Prime too. Whatever you’re buying from the big box pet store that says “water conditioner” will probably work fine.
- Ammonia: I use Dr. Tim’s Ammonium Chloride Solution for Fishless Cycling. I can accurately calculate and control how much ammonia I’m putting into my tank. Ammonia is important because it’s the catalyst that fuels the process. We need ammonia to grow the bacteria, so we can use the bacteria to consume ammonia from our tank critters!
- Water test kit: The API Freshwater Master Test Kit is the gold standard in the community and is what I use.
- Syringes: You don’t need this, but it sure makes it easier to hit that line on the test tube. I use some 20 ml syringes I got from Amazon. I can fill the syringe with tank water and then have enough to dispense into all four test tubes of the water test kit. I also have some smaller syringes and pipettes to measure things going into the aquarium.
- Test tube holders: Another nice-to-have is something to hold and store your test tubes in. I bought some random plastic thing from Amazon, but you can find them on Etsy too.
- Testing/dosing log: This can be as simple as a note on your phone, but I like to make a note of the dates I’m doing things and what I’m doing so I can refer back to it later. I log my test results and what I’m adding to the water.
Starting your cycle!
We’re now ready to start the nitrogen cycle in the aquarium.
- Set up the aquarium. Add substrate, rocks, driftwood, aquarium decorations, filter, heater, lights, and any plants you have on hand, if adding live plants.
- Add water and water conditioner.
- Test the water. When I’m starting my cycle, I like to test the water before adding the ammonia. I’ve tested the tap water before and found surprisingly high amounts of ammonia. This step is probably not needed by most, but I do it for peace of mind.
- Add ammonia. You’ll need to calculate how many mL of ammonia to add to your aquarium based on its size so that there is 2 ppm in the water. The instructions are on the bottle. Use the small syringe/pipettes to measure.
- Test the water again. I give my tank about 30 minutes, then I test the water again to verify the ammonia is at the right concentration. I only do this when I’m starting the cycle to verify I have the math right.
- Add beneficial bacteria. The best thing you can do is get some used filter media from an established tank if you have access to it. You can either squeeze the “gunk” from the used filter into your filter or, if you have the same filter, just use it directly! If you don’t have access to used filter media, then just wait. Dave at Aquarium Science has performed some tests and says the “bacteria in a bottle” products are ineffective for reducing the cycle time. I believe him.
- Wait!
Definition of ‘Cycled’
Your tank is cycled when it can convert 2 ppm ammonia to nitrates in 24 hours. That is, if you add enough ammonia to your tank to hit 2 ppm, when you test it the next day, it should have 0 ammonia, 0 nitrites, and some nitrates.
Monitoring the cycle
Immediately after setting up my tank, I do not perform my first water test until the 5th day or thereabouts. It takes a few days for the bacteria that eat ammonia to get established.
- I test every 2–3 days and try to keep the ammonia at ~2 ppm.
- If nitrite climbs into the 5–10 ppm range, the bacteria struggle to grow efficiently, and cycling can plateau. I will perform a water change.
- If nitrates go above 80 ppm, things can slow. I will perform a water change.
Sticking the landing and verifying results
After 3–4 weeks of testing your water every few days, your water tests will show the ammonia test as yellow (0!), nitrites as blue (0!), and nitrates as orange/red (some nitrates!). Rejoice because we’re very close now, and we just need to verify the results.
I like to dose 2 ppm again. If the test results are good after 24 hours, then I consider my tank cycled and I’m ready for fish. In my experience, I have to verify 2–3 days in a row before all nitrites are completely gone.
If it’s going to be a few days before I can add fish, I’ll dose the tank daily while it’s empty with a partial ammonia dose to keep the bacteria humming along.
The last 3 tanks I’ve cycled using this method have taken 3–4 weeks to complete.
Conclusion
That’s it! I hope someone who doesn’t know much about cycling their tank finds this post and finds it useful. Thanks for reading.
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