How to cycle a fish tank: fishless and fish-in, step by step

To cycle a fish tank, run the filter and heater with no fish, add pure ammonia to about 2 ppm, and keep re-dosing as the bacteria use it up, testing ammonia and nitrite every 1 to 3 days. The tank is cycled when a 2 ppm dose turns into 0 ammonia and 0 nitrite within 24 hours, with nitrate rising. That usually takes 4 to 8 weeks, or a few days to 2 weeks with filter media from a healthy established tank.

Cycling grows the bacteria that turn toxic ammonia into nitrite, and nitrite into much safer nitrate. A new tank has almost none of them, so fish waste builds up and poisons the fish. The nitrogen cycle guide explains the chemistry. This page is the practical plan.

You can cycle in two ways. In a fishless cycle, you feed the bacteria with bottled ammonia before any fish arrive. It is the safest method, so use it if you can. A fish-in cycle is the fallback when fish are already in the tank.

What you need before you start

A liquid test kit for ammonia, nitrite, nitrate and pH, plus a KH test if you can. Strips are vague at low ammonia levels.
Pure ammonia. Use ammonium chloride sold for fishless cycling, or household ammonia that lists only ammonia and water. If it foams a lot when shaken, it probably contains surfactants (soap). Never use a scented one.
Water conditioner that removes chlorine and chloramine, since both kill the bacteria.
The filter and heater running 24 hours a day. Set the heater to 76–82 °F (24–28 °C). Cold water slows the bacteria down.
Your real water volume. A “20 gallon” tank holds noticeably less once gravel and decor are in. The aquarium volume calculator gives you the figure to dose against.

Fishless cycle: the day-by-day plan

Day 0: set up and take baseline readings. Fill the tank, add conditioner, and run the filter and heater. Test the tank water for ammonia, pH and KH before you add anything. Chloramine is made from chlorine and ammonia, so treated tap water can show some ammonia once conditioned. Count it toward your 2 ppm.
Day 1: dose ammonia to 2 ppm. Add ammonia in small amounts, as described in the next section. Wait 30–60 minutes and test again. Stop at about 2 ppm, and never go above 5 ppm, because too much ammonia holds the bacteria back.
Days 2–10: wait for ammonia to fall. Test ammonia and nitrite every 2–3 days. Little seems to happen in the first week, and that is normal. Don’t add more ammonia until the reading drops below 0.5 ppm.
Days 7–14: nitrite appears. Nitrite showing up is proof that the first group of bacteria is working. From now on, test both every day or two. Each time ammonia falls below 0.5 ppm, add about half your first dose (about 1 ppm). Smaller doses keep nitrite from piling up while the second group of bacteria catches up.
Days 14–35: the nitrite phase. Ammonia now clears in a day or two, while nitrite climbs, often to the top of the test chart. Keep adding about 1 ppm each time ammonia is gone. This is the longest wait. Very high nitrite slows the bacteria you are waiting for, so if it reads 5 ppm or more, stop adding ammonia and do a 30–50% water change. Resume 1 ppm doses once nitrite is under 5 ppm.
Days 28–56: the 24-hour test. Once nitrite is falling steadily, go back to a full 2 ppm dose and test 24 hours later. If ammonia and nitrite both read 0 and nitrate is present, repeat the next day. Two clean passes in a row means the tank is cycled.
Before the fish arrive: lower nitrate. By now nitrate can be 40–100 ppm or more. Do one or two large water changes to get it under 20 ppm. If your tap water has no nitrate, going from 80 ppm to 20 ppm takes about a 75% change. The water change calculator does the math. Add fish within a day or two. If they are not coming yet, keep dosing 2 ppm every day or two so the bacteria don’t starve.

Check pH and KH once a week. The bacteria use up carbonate hardness (KH) as they work, so pH drifts down. Dr Tim’s Aquatics warns that they work slowly below pH 7.0 and the cycle can stall there. The Merck Veterinary Manual says alkalinity of 50 ppm (about 3 dKH) or less inhibits the biofilter, and suggests buffering to at least 100 ppm (about 5–6 dKH). A water change usually brings both back up, and crushed coral or a little plain baking soda, added gradually and retested, keeps them there. The unit converter switches between ppm and dKH.

How to dose ammonia accurately

Ammonium chloride made for cycling has a dose on the label, so follow it. Dr Tim’s Aquatics notes that household ammonia usually starts out at 4–11% and the bottle rarely says which, so work in steps:

Measure the ammonia with a needle-free syringe or a dropper marked in mL.
Add 0.5 mL per 10 gallons (38 L) of actual water.
Let the filter mix it for 30 minutes, then test.
Repeat until the test reads about 2 ppm, and write down the total. That total is your full dose for the rest of the cycle, and half of it is your 1 ppm dose for the nitrite phase.

As a rough guide, 1 mL of ammonia labelled 10% in 10 gallons (38 L) adds about 2 to 2.5 ppm, depending on how the label and your test kit express ammonia. Fish food or raw shrimp also releases ammonia, but you can’t control the level, so use them only as a last resort. During a fishless cycle, skip conditioners that bind or detoxify ammonia. They can confuse some ammonia tests, and Dr Tim’s Aquatics warns that overusing ammonia removers can stall the cycle.

Testing schedule

Stage	What to test	How often
Setup day	Ammonia, pH, KH (tap water and tank)	Once
After every ammonia dose	Ammonia	30–60 minutes later, to confirm the dose
Week 1	Ammonia and nitrite	Every 2–3 days
Weeks 2–6	Ammonia and nitrite	Daily or every other day
Every week	pH, KH and nitrate	Once a week
Final check	Ammonia, nitrite, nitrate	24 hours after a 2 ppm dose, 2 days running
Fish-in cycle	Ammonia and nitrite (pH weekly)	Every day, at the same time, before feeding

Many nitrate tests also react to nitrite, so nitrate can look alarmingly high mid-cycle. Trust the nitrate reading once nitrite is back to zero. To track readings over time, log them under a tank in My tanks.

What the readings usually look like

This is the usual pattern for a fishless cycle with no added bacteria, following the plan above. Your timing will differ, but the order doesn’t: ammonia falls, nitrite rises and then falls, and nitrate builds throughout.

Period	Ammonia	Nitrite	Nitrate
Week 1	Stays near 2 ppm, then starts to fall	0	0
Week 2	Falls faster, under 0.5 ppm within a few days	Appears, rising	Trace
Weeks 3–4	Gone within 24–48 hours	High, often at the top of the test chart	Rising
Weeks 5–6	Gone within 24 hours	Falling	High
Done	0 within 24 hours of a 2 ppm dose	0 within 24 hours	Rising after each dose

How long each cycling method takes

Plan on 4 to 8 weeks. Dr Tim’s Aquatics puts a fishless cycle without effective added bacteria at 30 to 45 days, the University of Florida extension service allows 6 to 8 weeks for a new biofilter, and the Merck Veterinary Manual says a tropical tank’s biofilter can take up to 8 weeks to establish.

Method	Typical time	Notes
Fishless, ammonia only	4–8 weeks	Most predictable
Fishless with bottled bacteria	Under 1 week to 6 weeks	Results vary by product; test to confirm
Seeded media from a healthy tank	A few days to 2 weeks	The fastest reliable shortcut
Fish-in cycle	4–8 weeks	Daily testing and frequent water changes
Cold water, pH below 7 or low KH	8+ weeks, or stalls	Fix conditions first

How to speed up the cycle

Seeded filter media is the best shortcut. A sponge, some ceramic rings or a used filter pad from a healthy tank that has run for a few months carries a ready-made colony. Keep it wet in tank water, put it in your filter the same day, and dose ammonia to 2 ppm. Some seeded tanks pass the 24-hour test within days. Only take media from a tank with no recent disease, because pests and parasites can come along with it. The Merck Veterinary Manual suggests a variation: run a clean sponge filter in an established, disease-free tank before you set up the new one.

Bottled bacteria are a maybe, not a promise. Some products contain live nitrifying bacteria and can cut the wait a lot. Others make little difference. One maker’s own FAQ says every tank cycles differently and that most are cycled within 4 to 6 weeks. Follow the label, turn off any UV sterilizer while you dose (UV kills the bacteria), and let the 24-hour test decide.

Get the conditions right. Keep the water at 76–82 °F (24–28 °C), pH between 7 and 8 (the best range for these bacteria) and KH at about 100 ppm (5–6 dKH). Keep a gentle ripple on the surface, since the bacteria need oxygen. Live plants help a little by taking up some ammonia, but they don’t replace a proper cycle.

Fish-in cycling: when the fish are already home

If you already have fish in an uncycled tank, don’t panic and don’t add more. The aim is to keep ammonia and nitrite low with water changes while the bacteria grow.

Keep the stock light. A common rule of thumb is about one small, hardy fish per 10 gallons (38 L). If you bought a full tank’s worth, be ready to change water every day.
Feed sparingly. Feed once a day, only what is eaten in about a minute. Skipping a day now and then is fine for healthy adult fish.
Test ammonia and nitrite every day. Test at the same time each day, before feeding.
Change water as soon as anything shows. Change water the same day ammonia or nitrite gives any reading (0.25 ppm on most kits). UF/IFAS notes nitrite can harm fish at as little as 0.1 ppm. If your tap water reads 0, a 50% change halves the reading. At 1 ppm, change about 75%. Match the temperature and use conditioner every time. A conditioner labelled to detoxify ammonia and nitrite can help between changes, but it doesn’t take them out of the water, so follow its label and keep changing water.
Watch the pH. The same ammonia reading is more dangerous in alkaline water. UF/IFAS tables show that at tropical temperatures around 77 °F (25 °C), about ten times more of the ammonia is in its toxic form at pH 8 than at pH 7. If your pH is 8 or higher, make each change big enough to bring ammonia back to 0, and test morning and evening.
Leave the filter alone. Don’t rinse or replace media during the cycle.
Add bacteria if you can. Seeded media or a live bottled product shortens the time the fish spend at risk.

The fish-in cycle is done when ammonia and nitrite stay at 0 for a week of normal feeding with no water changes in between, and nitrate is rising. Then add new fish a few at a time, and test for a week after each batch. The stocking calculator shows how many fish the tank can hold in the end.

New tank syndrome: signs and emergency fixes

New tank syndrome is fish poisoning from ammonia or nitrite in a tank that isn’t cycled yet. The Merck Veterinary Manual says it usually shows up within the first 6 weeks: fish that looked fine suddenly turn sluggish, stop eating and often die. Other signs are gasping at the surface, clamped fins, red or inflamed gills (ammonia burns gill tissue) and tan or brown gills (nitrite turns the blood brown, so fish suffocate even in well-oxygenated water). On the test kit, the giveaway is any ammonia or nitrite in a tank that is only weeks old.

What to do right away:

Test ammonia, nitrite and nitrate.
Change 50% of the water with conditioned water at the same temperature. Repeat daily until both read 0, then follow the fish-in steps above. If the tank’s pH has dropped a full point or more below your tap water, do smaller changes (about 25%) once or twice a day instead, because a sudden pH rise turns more of the remaining ammonia toxic.
Stop feeding for a day, then feed lightly.
Add seeded media or live bacteria.
For nitrite, adding chloride (aquarium salt) is a standard treatment, because the Merck Veterinary Manual notes chloride competes with nitrite at the gills. Follow the product label, check first that your fish and plants tolerate salt, and keep doing water changes, since salt doesn’t remove nitrite.
Add no new fish until the tank has passed a week of clean tests.

If fish are still gasping or dying after water changes, or keep dying once the water tests clean, contact an aquatic vet. Our ammonia guide covers emergencies in more detail.

What kills a cycle and what doesn’t

The bacteria live on surfaces, mainly the filter media, so ordinary water changes and gravel vacuuming are safe. These are the things that really hurt them:

rinsing filter media in chlorinated tap water instead of old tank water
replacing every sponge or cartridge at once
the filter being off for more than a few hours, since the bacteria run out of oxygen
some medications, especially antibacterial and copper-based treatments (check the label, and test ammonia and nitrite more often during treatment and for several weeks after)

The guide to cleaning a fish tank shows how to clean without losing the cycle.

Common mistakes

Mistake	What happens	Do this instead
Adding fish when the bottle says to	Ammonia spikes if the product didn’t take	Add fish only after the 24-hour test
Dosing far more than 2 ppm	Nitrite passes 5 ppm and the cycle stalls	Start at 2 ppm and drop to about 1 ppm once nitrite appears
Ignoring pH and KH	pH falls below 7, the bacteria slow down and the cycle can stall	Test weekly and buffer or change water
Stopping ammonia before the fish arrive	The colony shrinks, then the fish cause a spike	Keep dosing until the fish go in
Skipping the final water change	Fish go into 80+ ppm nitrate	Change water to under 20 ppm first

Gear mentioned in this guide

Liquid test kit (ammonia, nitrite, nitrate, pH) (opens Amazon in a new tab)Liquid kits are more accurate than strips. The API Freshwater Master Test Kit is the common choice. How to choose
Pure ammonia for a fishless cycle (opens Amazon in a new tab)Use ammonia with no soap or scent. Dose to about 2 ppm. How to choose
Bottled nitrifying bacteria (opens Amazon in a new tab)Can help a new tank cycle. Still test the water before adding fish. How to choose
Water conditioner (dechlorinator) (opens Amazon in a new tab)Removes chlorine and chloramine from tap water. Seachem Prime is a popular, concentrated choice. How to choose

Product links go to Amazon search results. As an Amazon Associate we earn from qualifying purchases, at no extra cost to you. Disclosure.

Frequently asked questions

How long does it take to cycle a fish tank?

Most fishless cycles take 4 to 8 weeks without help. Seeded filter media from a healthy tank can cut that to a few days or 2 weeks, while cold water, low pH or low KH can stretch it past 2 months.

How do I know when my tank is cycled?

Dose ammonia to 2 ppm, then test 24 hours later. If ammonia and nitrite both read 0 and nitrate is present, and that happens two days in a row, the tank is cycled.

Does a water change reset the cycle?

No. The bacteria live on the filter media, gravel and other surfaces, not in the water, so a water change removes almost none of them. Rinsing filter media in chlorinated tap water or replacing all of it at once can wipe them out, though.

Can I add fish after 24 hours if I use bottled bacteria?

Only if your tests agree. Some products work quickly, others barely change the timeline, and one maker's own FAQ says most tanks are cycled within 4 to 6 weeks. Dose ammonia to 2 ppm and add fish only once it clears to 0 ammonia and 0 nitrite within 24 hours.

Is a fish-in cycle cruel?

It is harder on the fish, which is why the fishless method is better. If fish are already in the tank, keep the stock small, feed lightly, test daily and change water the same day ammonia or nitrite shows any reading (0.25 ppm on most kits), so they are never left in toxic water.