How to
The fishless cycle, step by step
The whole point is that nothing is living in the tank while you do it. No fish is stressed, you can dose ammonia properly, and you end with a filter sized for a full stocking rather than one fish.
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We have not used any of this gear. Every pick below is chosen from published manufacturer specifications and the arithmetic on how we pick, which you can re-run yourself.
Set the tank up fully and get it warm, dose pure ammonium chloride to about 2 ppm, add a bacterial starter or seeded media, and test ammonia and nitrite every day or two — re-dosing ammonia to 2 ppm each time it hits zero. It is finished when the tank clears a full 2 ppm dose to zero ammonia and zero nitrite within 24 hours. Then do a big water change and add fish.
A fishless cycle is better than a fish-in cycle in every respect, and it is worth being explicit about why rather than treating it as the virtuous option.
- Nothing is harmed. No animal spends a month in water containing measurable ammonia.
- You can dose properly. With fish present you keep ammonia as low as possible; with no fish you can hold it at 2 ppm, which grows a much larger colony.
- You end up cycled for a full stocking, not for one hardy fish, because the colony sized itself to a real load.
- No water changes needed during the process, so it is genuinely less work.
- Mistakes cost nothing. Over-dose and you dilute and carry on. There is no one in there to hurt.
What to buy first
| Item | Why | Notes |
|---|---|---|
| Liquid test kit reading ammonia | The only way to see any of this | The common 6-in-1 strips do not test ammonia. See best aquarium test kits. |
| Pure ammonium chloride solution | The ammonia source | Must be free of detergents, surfactants, perfumes and colorants |
| Dechlorinator | Chlorine and chloramine kill the bacteria you are growing | Needed for the fill and every top-up |
| A bacterial starter, optional | Cuts four to six weeks to two or three | See do bacteria starters work |
| A notebook, genuinely | One reading means little; a column of them shows the shape | A phone note is fine |
Quick picks
The shopping list, in one place
| # | Product | Best for | Price |
|---|---|---|---|
| 1 | No image Various Pure ammonium chloride solution Pure ammonium chloride. The one ingredient a fishless cycle cannot be run without. | The ammonia source | |
| 2 | No image API Freshwater Master Test Kit Ammonia, nitrite and nitrate in fine increments, over 800 tests. | The only way to see any of this happening | |
| 3 | No image Seachem Prime Removes chlorine and chloramine, which kill the bacteria you are growing. | For the fill, and for every top-up after |
Rows link to the full write-up further down this page. Prices come from the live price layer only; where there is no live figure the button reads “Check price” rather than showing a remembered number.
Week zero: set up and start
- Assemble everything. Tank, substrate, filter, heater, decor, lid. Bacteria colonize surfaces, so everything that will be in the tank should be in it from the start.
- Fill with dechlorinated water and run the filter and heater for 24 hours. Confirm there are no leaks and the heater actually holds your target temperature — around 78 °F is a good cycling temperature and warmer speeds bacterial growth.
- Dose ammonia to about 2 ppm, then test to confirm you hit it. Do not assume the label arithmetic landed where you expected.
- Add your bacterial starter or seeded media now, so it has food from the moment it goes in.
- Write down the date and the ammonia reading. Day zero.
Weeks one to two: ammonia falls, nitrite appears
Test ammonia and nitrite every day or two. For the first several days, expect nothing to happen at all. Ammonia sits where you put it and nitrite reads zero. That is normal and it is where most people wrongly conclude the cycle has failed.
Then ammonia starts to fall and nitrite appears. This is the first real evidence, and it means the ammonia-oxidizing population has established. When ammonia reads zero, re-dose to 2 ppm — the colony is sized by its food supply and starving it now loses progress.
Nitrite will climb, often to alarming levels, sometimes off the top of the chart. Do not intervene. High nitrite is the expected middle of the process, not a problem to correct, and there are no fish in there for it to harm.
Weeks two to four: the nitrite plateau
This is the slow, discouraging part, and knowing it is normal is most of what gets people through it. Ammonia now clears to zero within 24 hours — the first half works. Nitrite sits high and does not seem to move, sometimes for a week or more.
Nitrite-oxidizing bacteria are genuinely slower to establish than the ammonia-oxidizing kind. Nothing is wrong. Keep dosing ammonia to 2 ppm whenever it clears, keep testing, and start watching for nitrate — its appearance is the first sign the second population is working.
Then nitrite falls, and when it goes it usually goes quickly: high one day, near zero two days later.
The finish test
Do not skip this. It is the only thing that distinguishes a cycled tank from an encouraging one.
- Dose ammonia to 2 ppm. A full dose, not a token one.
- Wait 24 hours. Not 12, not 36.
- Test ammonia and nitrite. Both must read 0 ppm.
- Test nitrate. It should be present, often high — that is the accumulated product of the whole month and it is exactly what you want to see.
- If either ammonia or nitrite shows anything, it is not finished. Re-dose, wait two days, test again.
After it passes
- Do a large water change, 50% or more, to bring the accumulated nitrate down. Dechlorinate it and match the temperature.
- Add fish within a few days. The colony needs feeding — cycle a tank and leave it empty for a fortnight and the bacteria starve back down.
- Add a few small fish, not the full stocking, even though the tank is cycled. See adding fish to a new tank.
- Keep testing for a fortnight. Ammonia and nitrite should stay at zero. If either appears, you added too many at once.
- Then settle into the routine — see how often to change aquarium water.
Troubleshooting
| Symptom | Likely cause | What to do |
|---|---|---|
| Nothing at all after 10 days | Cold tank, chlorine in the water, or an ammonia dose so high it inhibited the bacteria | Check the heater, confirm you dechlorinated, and dilute if ammonia is above 5 ppm |
| Ammonia falls, nitrite never appears | Usually it did appear and was already converted, in a well-seeded tank | Test nitrate. If it is present, the cycle is further along than you thought |
| Nitrite stuck high for over a week | Normal | Keep going. This is the expected plateau |
| Everything zero but nitrate is zero too | The ammonia dose never actually landed | Re-test your dosing. You may have added far less than you thought |
| pH crashed and the cycle stalled | Nitrification consumes alkalinity, and a low-buffered tank can drop far enough to halt it | A partial water change restores buffering and usually restarts it |
Already have fish in there? Go to why do my fish keep dying instead — the protocol is different and urgent.
Questions people actually ask
Frequently asked questions
+ What ammonia should I use for a fishless cycle?
+ How much ammonia should I dose?
+ How long does a fishless cycle take?
+ Should I do water changes during a fishless cycle?
+ My nitrite has been off the chart for a week. Is something wrong?
+ Can I use fish food instead of ammonia?
Where the numbers came from
Sources
- US EPA, Aquatic Life Criteria for Ammonia — toxicity, and its dependence on pH and temperature
- USGS, The Nitrogen Cycle — nitrification as a two-stage bacterial oxidation
- API Freshwater Master Test Kit — Amazon listing (parameters tested, test count, contents)
- Tetra SafeStart Plus — product page (bacterial genera, dosing, storage)
Read next
Related on Fin & Filter
How to cycle a fish tank
What cycling actually is, the fishless method start to finish, and the exact test result that means it is done.
Do bottled bacteria starters work?
What bottled bacteria can and cannot do, the storage problem nobody mentions, and how the products differ.
The best aquarium test kits
The kit that reads ammonia, the strips that quietly do not, and why that distinction decides everything.