How long does it take to cycle an aquarium?

A fishless cycle usually takes about 2–6 weeks. It's done when the tank can turn a dose of ammonia into zero ammonia and zero nitrite within 24 hours. Seeding it with filter media or substrate from an established tank, or using a quality bottled-bacteria product, can shorten it considerably.

Do I really need to cycle if I use a water conditioner?

Yes. A water conditioner only removes chlorine and chloramine from tap water — it does nothing to process the ammonia your fish produce. Cycling grows the living bacteria that actually detoxify waste. They are two completely different jobs.

What are safe ammonia, nitrite and nitrate levels?

In a cycled tank ammonia and nitrite should both read 0. Nitrate is far less toxic but still needs to be kept low with regular water changes — the RSPCA suggests keeping nitrate below 50 ppm. If you ever see ammonia or nitrite above 0 in a stocked tank, do a water change.

Can I speed the cycle up?

Yes — the fastest way is to 'seed' your new filter with mature media, gravel, or a sponge from an established, healthy tank, which transplants the bacteria directly. A quality bottled-bacteria starter also helps, and live plants take up ammonia from day one.

I already put fish in an un-cycled tank — what do I do now?

Switch to a fish-in cycle: test daily, and do a water change whenever ammonia or nitrite rises above 0 to keep the fish safe. Feed lightly, don't add any more fish until the tank is cycled, and use a conditioner that also detoxifies ammonia as a stop-gap. It's slower and more work than fishless — but it will get you there without losing the fish.

The nitrogen cycle: the one thing nobody told you before you bought fish

The 30-second version. Your fish constantly produce ammonia, which is toxic. A new tank has no way to deal with it — so fish added to a fresh tank are slowly poisoned. “Cycling” means growing a colony of invisible bacteria that turn that ammonia into far safer nitrate before any fish arrive. Get the cycle right and most beginner disasters simply never happen. Get the water right, and the fish look after themselves.

If you remember one thing from this entire website, make it this page. The nitrogen cycle is the single most important idea in fishkeeping, and it’s the one thing the shop that sold you the tank almost certainly skipped. Nearly every “my fish died and I don’t know why” story traces back to it.

The good news: it’s genuinely simple once someone explains it. Nobody did. So here we go — kindly, and in plain English.

🩺 What the fish vets say: “Water quality is the #1 factor in influencing fish health.” — Dr. Jessie Sanders, DVM, DABVP (Fish Practice), in Fish Water Quality Basics. The nitrogen cycle is how you get that water quality right.

The problem: a new tank is trying to poison your fish

Fish are living animals in a closed box of water. They breathe, eat, and produce waste — and that waste, along with any uneaten food, releases ammonia into the water. Fish also excrete ammonia directly through their gills, all day, every day.

Ammonia is extremely toxic to fish. Even small amounts burn their gills, stress them, and — given time — kill them. In a river or lake, this never matters: the water is endless and always moving. In your tank, there’s nowhere for it to go.

So the whole game of keeping fish is really one question: what happens to the ammonia? In a healthy tank, the answer is a tiny, invisible cleanup crew.

Meet the cleanup crew: the nitrogen cycle

The nitrogen cycle is a chain of beneficial bacteria that live on every surface in your tank — especially inside your filter. They do the detox work for you, in three steps:

Ammonia → nitrite. One group of bacteria (Nitrosomonas) eats the toxic ammonia and turns it into nitrite.
Nitrite → nitrate. Unfortunately, nitrite is also toxic. But a second group of bacteria (Nitrobacter / Nitrospira) eats the nitrite and turns it into nitrate.
Nitrate → removed by you. Nitrate is far less harmful. It builds up slowly, and you export it with regular water changes (and live plants use some of it as fertiliser).

That’s it. That’s the cycle: ammonia → nitrite → nitrate → water change. Two toxins, converted by bacteria into one much safer compound that you remove by hand.

A tank with a mature, working cycle is a tank where ammonia and nitrite always read zero, because the bacteria consume them as fast as the fish produce them. That’s the goal — and it’s why we say fishkeeping is really water-keeping.

Why “new tank syndrome” kills so many first fish

Here’s the catch: those bacteria are not in a brand-new tank. They have to grow, and grow slowly, over a few weeks. A filter fresh out of the box is just a pump — the biology that makes it a life-support system isn’t there yet.

So when someone sets up a tank on Saturday, adds fish on Saturday afternoon, and finds them sick or dead a week or two later, this is almost always what happened:

Day 1–3: fish produce ammonia. There are no bacteria to process it, so it climbs. The fish are being poisoned.
Week 1–2: the first bacteria finally appear and ammonia starts to fall — but now nitrite spikes, because the second group of bacteria hasn’t caught up. Nitrite is toxic too, so the fish take a second hit.
Week 3–6: the second group catches up, nitrite falls to zero, and nitrate begins to appear. The tank is finally “cycled” — but the first fish often didn’t survive the wait.

This slow poisoning of the first fish in a fresh tank is so common it has a name: New Tank Syndrome. It isn’t bad luck, and it isn’t your fault — it’s a missing step nobody told you about.

The kind way to do it: fishless cycling

The fix is beautifully simple: grow the bacteria before the fish arrive. This is called fishless cycling, and it means no animal ever has to suffer through the ammonia and nitrite spikes.

The idea is to feed the empty tank a source of ammonia yourself, so the bacteria have something to eat and multiply on, then wait until the tank can clear it:

Set up the tank fully — filter running, heater on (bacteria grow faster in warm water), dechlorinated water, substrate and plants in. Buy a liquid test kit (test strips are fine for a quick look, but a liquid kit is far more reliable for cycling).
Add an ammonia source. A bottle of pure household ammonia (no additives, no perfume) dosed to about 2–3 ppm is the classic method. Adding a pinch of fish food to rot, or a quality bottled-bacteria product, also works.
Test and wait. Over the coming weeks you’ll watch ammonia rise then fall, then nitrite rise then fall, then nitrate appear. Keep topping the ammonia back up to ~2 ppm so the growing colony doesn’t starve.
You’re cycled when the tank can take a ~2 ppm dose of ammonia and turn it into 0 ammonia and 0 nitrite within 24 hours. Do a big water change to bring the nitrate down, and now add your fish.

Want to skip weeks of waiting? Seed the new filter with a used sponge, some gravel, or filter media from an established, healthy tank — you’re literally transplanting a ready-made bacterial colony. This is the single biggest shortcut there is.

Already have fish in an un-cycled tank? Don’t panic

Lots of people find this page after the fish are already in. That’s okay — you can still get there without losing them, with a fish-in cycle:

Test the water every day for ammonia and nitrite.
Do a water change whenever either rises above 0 — enough to bring it back down. This is you doing the bacteria’s job by hand until they take over.
Feed lightly (less food = less ammonia) and do not add any more fish until the tank is fully cycled.
Use a conditioner that detoxifies ammonia (e.g. a Prime-type product) as a temporary safety net between water changes.

It’s more work and slower than fishless cycling, but it’s the humane rescue when fish are already relying on you.

Keeping the cycle healthy for good

Once your tank is cycled, the bacteria look after themselves — as long as you don’t accidentally starve or poison them:

Never rinse your filter media in tap water. Chlorine kills the bacteria living in it. Rinse it gently in a bucket of old tank water instead, and only when flow drops.
Don’t replace all your filter media at once — that’s where most of your colony lives. Swap parts gradually.
Add fish slowly. A big sudden increase in stock is a big sudden increase in ammonia the colony hasn’t grown to handle yet.
Keep testing occasionally, and keep up regular water changes to keep nitrate low. The RSPCA suggests keeping nitrate below 50 ppm.

Do that, and you’ll have crossed the line that separates the people whose fish mysteriously die from the people whose fish just… thrive. It was never about luck. It was always about the water.

Sources & further reading

INJAF — The Nitrogen Cycle and the Fishless Cycle — a non-commercial fishkeeping-education charity’s beginner guide to the cycle and fishless cycling.
Aquarium Co-Op — The Beginner’s Guide to the Nitrogen Cycle — the ammonia→nitrite→nitrate process and the “2 ppm → 0 in 24 hours” completion test.
RSPCA Knowledgebase — Caring for goldfish — water-quality targets (ammonia < 0.1, nitrite < 0.2, nitrate < 50 ppm) and the recommended testing cadence for a new tank.