How to cycle a fish tank: the nitrogen cycle, fishless cycling and fish-in cycling explained

Cycling a fish tank means growing the helpful bacteria that turn toxic fish waste into much safer nitrate, and it takes weeks, not days. The single most important rule: don't fill a new tank with fish until a test kit shows ammonia and nitrite are being cleared to zero, because "new tank syndrome" is one of the biggest killers of beginners' fish.

Here's the nitrogen cycle in plain terms. Fish and rotting food make ammonia, which burns gills. One group of bacteria eats ammonia and makes nitrite, which is also toxic. A second group eats nitrite and makes nitrate, which is far less harmful and is removed with water changes and by plants (UF/IFAS). These bacteria live mostly on surfaces in your filter, not in the water. In a brand-new tank they don't exist yet. UF/IFAS says a tank's biofilter takes six to eight weeks to establish.

How to identify it

Signs your tank isn't cycled yet:

The tank was set up in the last few weeks, or the filter is new.
Ammonia or nitrite reads above zero on a test.
The water turns cloudy white or grey in the first weeks (a bacterial bloom; see cloudy water).
Fish gasp, hang at the surface, lose colour or die one by one in the first 1–3 weeks. The Merck Veterinary Manual describes new tank syndrome as appearing within 1–3 weeks of setup.

What a cycle looks like on your tests: ammonia rises first, then falls as nitrite rises. Then nitrite falls and nitrate climbs. Merck says the process is complete once nitrite returns to normal. In one study of new freshwater tanks, nitrite peaked around day 21–22 and was gone by about day 26–38 (Hovanec and others, 1998).

Easily confused with: an ammonia spike in an established tank (same chemistry, different cause), chlorine damage, and disease. Testing tells you which.

When it's a problem

Normal, don't worry: ammonia and nitrite readings in a new tank with no fish. That's the cycle working. Brown diatom dust and a cloudy bloom in the first month are also normal.
Early warning: fish in a tank under 6–8 weeks old with ammonia or nitrite at 0.25–0.5 mg/L (ppm). Manageable with water changes.
Serious: ammonia or nitrite at 1 mg/L or more with fish in the tank. The Merck manual advises at least a 50% water change when total ammonia reaches 2 mg/L or higher.
Emergency: fish gasping, lying on the bottom, or dying. Go to the next section now.

If you already have fish in an uncycled tank

Many people learn about cycling this way. Don't panic. This is "fish-in cycling", and you can do it safely with daily care.

Test ammonia and nitrite today. Use a liquid test kit if you can.
If either is 0.5 mg/L or higher, do a 30–50% water change with dechlorinated, temperature-matched water.
Add a water conditioner that detoxifies ammonia and nitrite at each change. It binds them temporarily (about a day) while still letting bacteria use them.
Feed very lightly: once a day or every other day, only what's eaten in a minute or two. Remove leftovers.
Don't add more fish until the cycle is done.
Don't clean or replace the filter. It's where the new bacteria are growing.
Test every day, and change water whenever ammonia or nitrite reaches about 0.5 mg/L. Expect to do this for several weeks.
Consider seeding (see below): a handful of used filter media from a healthy tank can cut weeks off.

What to do next

Fishless cycling (the kindest way)

Fishless cycling grows the bacteria before any fish go in, using a non-fish ammonia source. Wikipedia's summary notes that pure ammonia works better than rotting fish food, which also adds phosphate.

Set up the tank fully: dechlorinated water, filter running, heater at about 24–26°C (75–78°F).
Add an ammonia source: pure ammonium chloride sold for aquarium cycling, or plain household ammonia with no scent, soap or surfactant (shake it: if it foams, don't use it).
Aim for about 2 mg/L (ppm) ammonia. Test 30 minutes after dosing. Products differ in strength, so test rather than trust the math. One cycling guide advises never letting ammonia go above 5 mg/L (Bulk Reef Supply); too much ammonia can stall the cycle by harming the bacteria you're trying to grow.
Test ammonia and nitrite every day or two. When ammonia drops, top it back up to about 2 mg/L. Don't add more if ammonia or nitrite is already high.
Watch nitrite rise and then fall. This second stage is often the slowest.
The 24-hour test: your tank is cycled when it turns about 2 mg/L of ammonia into zero ammonia and zero nitrite within 24 hours, with nitrate rising.
Do a large water change to bring nitrate down, then add fish.

How long it takes: usually several weeks; UF/IFAS gives six to eight weeks for a tank biofilter to establish. Wikipedia notes it can be very fast with seeding, or take months in some reported cases. Warmth helps, but very high pH and temperature aren't always better, since some nitrifying bacteria prefer moderate conditions.

Seeding with mature filter media

The fastest, cheapest boost is bacteria that already exist. Put a used sponge or a handful of ceramic rings from a healthy, disease-free tank into your new filter, kept wet in tank water on the way. Friends, clubs and good fish stores often share. Only take media from a tank you trust: it can carry parasites and snails too.

Bottled bacteria: what it really does

Results are mixed. A 1998 study found that the bacteria that actually clear nitrite in freshwater tanks are mostly Nitrospira-type, not the Nitrobacter found in the commercial product they tested. That product never established itself, though treated tanks cleared nitrite somewhat sooner, perhaps by feeding the natural bacteria (Hovanec and others). A later comparison cited by Wikipedia found only one of several products sped up cycling. Honest summary: some help a bit, many don't, and none replaces testing.

Stocking after the cycle

Add fish a few at a time, a week or two apart, and test after each addition. The bacteria grow to match the waste load.

Mini-cycles in established tanks

A cycled tank can partly "uncycle". Common causes:

Cleaning the filter too hard: replacing all the media, or rinsing it under chlorinated tap water. Rinse media gently in old tank water, and never replace more than about half at once.
Medicines: UF/IFAS warns some antibiotics and disease treatments can harm the filter bacteria. Test more often after treatment.
Chlorine, a power failure that starves the filter of oxygen, or a pH crash (the bacteria stop working at very low KH and pH).

Treat a mini-cycle exactly like fish-in cycling: test daily, change water, feed lightly.

Testing: liquid kits vs strips

Liquid kits (drops in a tube) take a few minutes but read low ammonia and nitrite levels more clearly than strips, which are handy for a rough check but hard to read at low levels. UF/IFAS notes that some ammonia test methods (Nessler-type) can give false readings after certain water conditioners; salicylate-type ammonia tests are preferred.

Cautions: add shrimp, snails and sensitive fish (many plecos, loaches, scaleless fish) only after the tank has been cycled and stable for a few weeks.

Products that help

Liquid ammonia test kit (salicylate type): the most important tool for cycling; use it daily during a cycle.
Liquid nitrite and nitrate test kits: show the second and third stages of the cycle.
Water conditioner that removes chlorine and chloramine and detoxifies ammonia: for every water change, and for fish-in cycling.
Pure ammonium chloride for cycling (or unscented household ammonia): the ammonia source for fishless cycling.
Bottled nitrifying bacteria: may speed things up a little; not a substitute for testing.
Sponge filter or porous ceramic media: lots of surface for bacteria; a spare sponge kept running in an established tank gives you instant seed for a new or hospital tank.

Keep on hand: liquid ammonia and nitrite tests, a spare bottle of conditioner, and a sponge filter "seeding" in your main tank.

How to prevent it

Cycle every new tank before adding fish, and stock slowly.
Keep a spare sponge filter running in your main tank so any new tank starts cycled.
Clean filters gently, in tank water, never all at once. Test weekly, and after any medicine, cleaning or power cut.

Why it happened

Usually fish went into a brand-new tank too soon. Other times a filter was scrubbed or replaced, a medicine harmed the bacteria, or too many fish were added at once.

When to get help

Ask a good local fish store or aquarium club if your cycle stalls for more than a few weeks, if your tap water already contains ammonia, or if fish keep dying despite daily water changes. A vet who sees fish can help if sores or infections follow. Honest limit: fish badly burned by ammonia may die even after the water is fixed.

In the game

Cycling isn't a card in Multiple Tank Syndrome, but the Ammonia spike card is what happens without it, and the Better filtration and Quarantine tank cards are the closest real-life fixes.