01
What the nitrogen cycle in an aquarium is
Fish excrete ammonia through their gills and in their waste, and uneaten food and dead plant leaves break down into ammonia too. In a closed glass box, that ammonia has nowhere to go. Even 0.5–1 ppm can burn gills and fins, which is why so many fish die in brand-new tanks, a problem often called new tank syndrome.
The aquarium nitrogen cycle is the process that fixes this. Two groups of microbes colonize every surface of the tank, above all the filter media. Ammonia-oxidizing bacteria and archaea (the group long credited to Nitrosomonas) turn ammonia into nitrite. Nitrite-oxidizing bacteria, mainly Nitrospira, turn nitrite into nitrate. Nitrate is far less toxic and is removed by plants and by regular water changes.
"Cycling" a tank simply means growing enough of these bacteria to handle the waste your fish will produce, before the fish suffer from it. The bacteria live on surfaces, not in the water, so a tank full of fresh water is not cycled no matter how clear it looks.
- Ammonia (NH3/NH4+): highly toxic, produced by fish, food and decay
- Nitrite (NO2-): highly toxic, blocks oxygen transport in the blood
- Nitrate (NO3-): low toxicity, keep below 20–40 ppm with water changes
02
Why cycling matters before you add fish
Ammonia toxicity depends on pH and temperature. Above pH 7, more of it exists as free ammonia (NH3), the form that damages tissue, so the same test reading is more dangerous in hard, alkaline water. Nitrite is just as harmful: it enters the blood through the gills and turns hemoglobin into a form that cannot carry oxygen. Fish gasp at the surface even when the water is well aerated. Our page on ammonia poisoning in fish covers the symptoms and emergency steps.
A cycled tank keeps both readings at zero around the clock, which no schedule of water changes can do on its own. It is the foundation for everything else in our beginner aquarium guide: stable water first, fish second.
03
What you need to cycle a tank
You do not need special equipment, but two items make the difference between guessing and knowing: a liquid test kit and a reliable ammonia source.
- A running filter: the media is where most bacteria will live. Keep it running 24/7 from day one.
- A heater: nitrifying bacteria grow fastest around 77–86 °F (25–30 °C). Below 64 °F (18 °C) the cycle crawls.
- A liquid test kit for ammonia, nitrite, nitrate and pH. Strips are fine for a quick check but less precise; see our best aquarium test kit comparison.
- A water conditioner that removes chlorine and chloramine, since both kill bacteria. Our best water conditioner guide compares the main options.
- An ammonia source: pure ammonium chloride made for fishless cycling (such as Dr. Tim’s Ammonium Chloride), household ammonia with no surfactants or scent, or fish food.
- Optional: bottled nitrifying bacteria (FritzZyme TurboStart 700, Dr. Tim’s One & Only, Tetra SafeStart Plus, Seachem Stability) or mature media from a healthy tank.
04
How to do a fishless cycle step by step
The fishless cycle is the method most experienced keepers recommend. No animal is exposed to ammonia, you can push the levels higher to grow a larger bacterial colony, and you can take your time.
- Set up the tank completely: substrate, decor, plants, conditioned water, filter and heater. Set the temperature to 78–82 °F (25.5–28 °C).
- Test the tap water first so you know your baseline pH, KH and whether it already contains ammonia (some chloramine-treated water does).
- Dose ammonia to about 2 ppm, following the product label. Test 30–60 minutes later to confirm. Avoid going above 4 ppm: very high ammonia can slow the nitrite-eating bacteria and stall the cycle.
- If you use bottled bacteria, add it the same day, after the conditioner, and follow the label.
- Test ammonia and nitrite every 2–3 days. When ammonia falls below 1 ppm, top it back up to 2 ppm. Keep feeding the bacteria; if they starve for more than a few days, the colony shrinks.
- Watch for nitrite. It usually appears in week 1–3 and can climb off the chart. This is normal. Keep dosing ammonia to 2 ppm whenever it drops.
- Watch for nitrate. When it appears and nitrite starts to drop, the second group of bacteria is establishing.
- Run the 24-hour test: dose 2 ppm ammonia, wait 24 hours, then test. If ammonia and nitrite both read 0 ppm, the tank is cycled. Repeat for 2–3 days in a row to be sure.
- Do a large water change (50–80%) to bring nitrate down below 20 ppm, then add your fish within a day or two. If you wait longer, keep dosing ammonia so the bacteria do not starve.
Stock gradually even after a full cycle. The colony is sized for the ammonia you dosed, so add fish in batches over a few weeks and keep testing for the first month.
05
Nitrogen cycle timeline: what to expect week by week
Every tank is different: temperature, pH, KH and the amount of seeded bacteria change the pace. Use this timeline as a typical pattern for a fishless cycle without bottled bacteria, not a deadline.
| Period | What happens | Typical readings |
|---|---|---|
| Days 1–7 | Ammonia sits high, bacteria are just starting to colonize | Ammonia 2 ppm, nitrite 0, nitrate 0 |
| Weeks 1–3 | Ammonia starts to fall, nitrite appears and rises | Ammonia dropping, nitrite 1–5+ ppm |
| Weeks 2–5 | Ammonia is processed within 1–2 days, nitrite peaks | Ammonia 0–0.5, nitrite high, nitrate rising |
| Weeks 3–8 | Nitrite falls quickly once Nitrospira catch up | Ammonia 0, nitrite falling, nitrate 20–80 ppm |
| End of cycle | 2 ppm ammonia cleared in 24 h, no nitrite left | Ammonia 0, nitrite 0, nitrate present |
06
How to read your test results
Your test kit tells you exactly where you are in the cycle. Read the three main values together rather than one at a time.
| Ammonia | Nitrite | Nitrate | What it means |
|---|---|---|---|
| High | 0 | 0 | Cycle not started yet; be patient, check temperature |
| Falling | Rising | 0 or trace | First bacteria are working; cycle is on track |
| 0 | High | Rising | Second stage; keep feeding ammonia, do not change the media |
| 0 | 0 | Rising | Cycled, if it holds after a 2 ppm dose for 24 h |
| 0 | 0 | 0 | Nothing to process; dose ammonia, or the tank was never fed |
Some ammonia tests read slightly above zero in tap water treated with chloramine or with certain conditioners. Test your tap water alone to know your baseline.
07
Fish-in cycling: how to do it safely
Sometimes the fish come first: a gift, an impulse buy, or a tank set up before anyone mentioned the cycle. A fish-in cycle works, but it puts the burden on you to keep ammonia and nitrite low while the bacteria grow. Expect it to take 4–8 weeks.
- Keep the stocking light: a few small, hardy fish for the tank size, never a full community.
- Feed sparingly, once a day, only what is eaten in a minute.
- Test ammonia and nitrite every day.
- Change 25–50% of the water whenever ammonia or nitrite reaches 0.25–0.5 ppm. Match the temperature and always use conditioner.
- Use a conditioner that binds ammonia and nitrite, such as Seachem Prime, to reduce their toxicity between water changes. It does not remove them, so keep testing.
- Add bottled bacteria to shorten the process.
- Do not clean or replace the filter media during the cycle; at most, rinse it gently in removed tank water if the flow drops.
08
Ways to speed up cycling
The fastest method is seeding with mature media: a used sponge, a handful of ceramic rings or filter floss from a healthy, disease-free tank. Squeeze or place it in your filter and the cycle can be done in days instead of weeks. Ask a friend or a trusted local fish store, and only take material from a tank without recent disease.
Bottled bacteria products also help. Quality varies, and results depend on handling and storage, so test as if you had not used one. Live plants help indirectly: they absorb ammonia and add surface area, though a heavily planted tank can make readings confusing. Keep the temperature in the upper range and the pH above 7 if your water allows it.
09
Troubleshooting a stalled cycle
A cycle is "stalled" when readings stop changing for a week or more, most often with nitrite stuck high or ammonia that will not budge. In almost every case, one of the causes below is responsible.
| Cause | Sign | Fix |
|---|---|---|
| pH below about 6.5 | Ammonia stops dropping, pH has crashed over the weeks | Raise KH gradually (baking soda or a KH buffer), aim for pH 7–8 during cycling |
| KH used up | KH reads 0–1 °dH, pH unstable | Water change with tap water, add buffer, test KH weekly |
| Too much ammonia | Ammonia or nitrite above 5 ppm for weeks | Water change to dilute, then dose only to 2 ppm |
| Low temperature | Tank below 70 °F (21 °C) | Use a heater, aim for 78–82 °F (25.5–28 °C) |
| Chlorine or chloramine | Progress resets after water changes | Dose conditioner for the full tank volume every time |
| Medication or antibiotics | Cycle stopped after a treatment | Water changes, carbon to remove residues, reseed bacteria |
| Media cleaned or replaced | Ammonia spikes after maintenance | Rinse media only in tank water, never replace it all at once |
A pH crash is the most overlooked cause. Nitrification produces acid and consumes KH, so soft tap water can drop to pH 6 or lower midway through the cycle. Test pH and KH once a week.
10
Common cycling mistakes
Most of these come from impatience. The cycle cannot be skipped, only shortened, so treat the first month as part of building the tank.
- Adding a full load of fish on day one because the water looks clear
- Testing only ammonia and declaring the tank cycled before nitrite has dropped
- Replacing filter cartridges during the cycle and throwing the bacteria away
- Letting the bacteria starve for a week between the end of the cycle and buying fish
- Using scented or soapy household ammonia, which can poison the tank
- Assuming cloudy water is a sign of failure: a bacterial bloom is common early on and usually clears by itself, see cloudy fish tank water
11
When to escalate and what to buy
If fish in a cycling tank gasp at the surface, sit on the bottom with clamped fins or show red, inflamed gills, do a 50% water change right away, dose a binding conditioner and test again. If the readings are fine but fish still decline, the problem may not be the cycle; check our troubleshooting hub and consider an aquatic veterinarian for serious cases.
Once the tank is cycled, keep it that way: weekly aquarium water changes, gentle filter maintenance and steady water parameters. If pH drifts out of range later, see how to lower pH or raise pH safely. The two products worth buying first are a liquid test kit and a good conditioner; a solid aquarium filter with plenty of biological media makes the whole process easier.
Frequently asked questions
Frequently asked questions
How long does it take to cycle a fish tank?
A fishless cycle usually takes 4–8 weeks. With bottled bacteria and a warm tank around 80 °F (27 °C), 2–4 weeks is common, and seeding with mature filter media from a healthy tank can cut it to a few days. Low pH, cold water or chlorinated top-ups can stretch it to 10 weeks or more.
How do I know when my fish tank is cycled?
Dose ammonia to 2 ppm and test 24 hours later. If ammonia and nitrite both read 0 ppm and nitrate is present, the tank is cycled. Repeat the test two or three days in a row to confirm, then do a large water change to lower nitrate before adding fish.
Is a fishless cycle better than a fish-in cycle?
Yes, in most cases. A fishless cycle exposes no animal to ammonia or nitrite, lets you grow a larger bacterial colony and needs fewer water changes. A fish-in cycle is a valid fallback when fish are already in the tank, provided you test daily and change water whenever ammonia or nitrite reaches 0.25–0.5 ppm.
Why is my nitrite so high and not going down?
High nitrite during weeks 2–5 is normal: the nitrite-eating bacteria grow more slowly than the ammonia-eating ones. If it stays stuck for over a week, check pH and KH first, since a pH below about 6.5 slows them sharply. Also avoid overdosing ammonia; a 50% water change and a steady 2 ppm dose usually restart progress.
Do water changes slow down the nitrogen cycle?
Not if you use conditioner and match the temperature. The bacteria live on the filter media and surfaces, not in the water, so a water change does not remove them. During a fishless cycle you rarely need one, except to bring down very high ammonia or nitrite, or to restore pH and KH.
Can I add fish as soon as the water is clear?
No. Clear water says nothing about ammonia or nitrite. A new tank can look perfect and still have zero bacteria. Wait until the 24-hour test passes, then add fish gradually, a few at a time over several weeks, so the bacterial colony can grow with the bioload.
Sources and references
Written and reviewed by the handsoffmyhobby editorial team according to our editorial policy.
