How to cycle a new aquarium

The single most important step before adding fish — explained from first principles, with the methods, numbers, and timeline you actually need.

Published 2 June 2026 · Last updated 2 June 2026 · 12 min read

What "cycling" actually means

When fish live in water, they continuously excrete ammonia through their gills and waste. Ammonia is highly toxic to fish — even small amounts cause gill damage, stress, and at higher concentrations, death within hours. A new tank filled with tap water has no way to remove ammonia. Every fish placed into an uncycled tank is being poisoned slowly by its own waste.

Cycling is the process of growing two specific types of beneficial bacteria inside your filter and on every surface of the tank. These bacteria do the detoxification work for you:

• Nitrosomonas bacteria convert ammonia into nitrite. Nitrite is still toxic, just slightly less so.
• Nitrobacter and Nitrospira bacteria convert nitrite into nitrate. Nitrate is far less toxic and is removed by water changes or absorbed by live plants.

The full pathway looks like this:

Fish waste → Ammonia (toxic) → Nitrite (toxic) → Nitrate (manageable)

This is the nitrogen cycle. A tank is "cycled" when it has enough of these bacteria to convert all the ammonia your fish produce into nitrate, in real time, with no toxic residue. Until that point, adding fish causes them measurable harm.

Why most beginners get this wrong

Pet shops routinely tell new customers their tank is ready after sitting with water for a few days, or that they can "cycle with fish" by adding hardy species first. Both pieces of advice are wrong, and both kill fish.

A tank with water but no bacteria is not cycled — it's just full of water. The bacteria need an ammonia source to colonise; they don't appear from nothing. And cycling with fish — sometimes called fish-in cycling — uses living animals as the ammonia source, exposing them to days or weeks of toxic conditions. It does work in the sense that the cycle eventually establishes, but at the cost of fish welfare and frequently fish lives. It's considered ethically substandard practice in modern fishkeeping.

The right approach is fishless cycling: provide a controlled ammonia source yourself, let the bacteria grow without any fish present, and only add livestock once the tank is processing ammonia to safe levels.

What you'll need

• A fully set up aquarium with substrate, filter running, heater set to your intended species' temperature range, and dechlorinated tap water
• A liquid freshwater test kit that measures ammonia, nitrite, nitrate, and pH — the API Freshwater Master Test Kit is the most widely-used reference. Avoid test strips; they are inaccurate at the precise low concentrations you need to read.
• An ammonia source — either pure ammonium chloride or plain fish food (covered in detail below)
• Patience — a fishless cycle typically takes 4 to 6 weeks. Anyone promising 24-hour or 3-day cycling is selling you something.

Optional but useful

• A bottled bacteria starter (Dr Tim's One and Only, Seachem Stability, or Fritz TurboStart). These genuinely accelerate the cycle when stored and used correctly. Older bottled products sat on shelves too long to be effective; the modern refrigerated formulations are different.
• Filter media or substrate from an established tank, if you know another fishkeeper. A handful of mature filter media is the fastest cycle accelerator that exists — sometimes shortcutting weeks to days.
• Live plants. Plants directly absorb ammonia and nitrate and bring some bacteria with them. A heavily planted tank cycles faster and more gently.

Method 1: Pure ammonia (recommended)

This is the most precise and reliable method. You dose a measured amount of pure ammonia, test daily, and watch the cycle progress.

Step 1 — Source pure ammonia

You need ammonium chloride (NH₄Cl) with no surfactants, perfumes, or additives — the kind sold specifically for aquarium cycling. Dr Tim's Ammonium Chloride and Fritz Fishless Fuel are the standard products. Household cleaning ammonia is not a safe substitute unless you can confirm it's pure ammonia with no other ingredients, which most aren't.

Step 2 — Dose to roughly 2 ppm ammonia

The target is 2 ppm of ammonia in the tank water. Higher concentrations (4 ppm and above) can actually stall the cycle by inhibiting the very bacteria you're trying to grow. The bottle of ammonia will give you a dosing chart based on your tank size — typically a few drops per litre. Follow the manufacturer's chart for your specific product.

After dosing, wait an hour for the ammonia to mix evenly through the water, then test. Adjust if needed. Re-record your dose so you can match it next time.

Step 3 — Test every day, record everything

For the next several weeks, test ammonia, nitrite, and nitrate daily. Keep a simple log — a sticky note on the tank works fine — with date and the three readings. You're looking for a specific pattern.

For the first week or so, ammonia will stay high and nitrite will read zero. The Nitrosomonas bacteria are still establishing.

Somewhere around week two, ammonia readings will start to drop and nitrite will begin to climb. This is the first sign the cycle is progressing — the ammonia is being converted.

Nitrite will then spike, often to very high readings (5 ppm or higher). The second bacteria population — the nitrite-converters — takes longer to establish. This nitrite spike is the longest stage of the cycle and is the part where impatient fishkeepers give up.

Eventually nitrite will start to fall while nitrate climbs. When both ammonia and nitrite read zero, and nitrate is detectable, the bacteria are doing their job.

Step 4 — Keep the bacteria fed

Once ammonia readings start dropping, redose to 2 ppm. The bacteria need a continuous food supply or they'll die back and you'll have to start over. Redose whenever ammonia drops to roughly zero, typically every one to three days as the cycle progresses.

Step 5 — The final test

Your tank is cycled when, after dosing 2 ppm of ammonia, both ammonia and nitrite read zero within 24 hours. This is the proof that your bacteria can process a meaningful waste load fast enough to keep fish safe.

Do this test twice in a row over two days before declaring victory. A single zero reading can be a fluke; two consecutive successful tests confirms the cycle.

Method 2: Fish food (slower but no ammonia purchase)

If you can't source pure ammonia, plain fish food works as an ammonia source. As the food decomposes, it releases ammonia into the water naturally. This method is slower and less precise, but it works and uses materials you already have.

How it works

Drop a small pinch of flake or pellet food into the empty tank every day, as though you were feeding invisible fish. The food sinks, decomposes, and releases ammonia gradually. The same nitrogen cycle establishes; it just takes longer to reach the ammonia concentrations needed to drive bacterial growth.

The tradeoffs

Fish food cycling typically takes 6 to 8 weeks, sometimes longer. The ammonia levels are unpredictable — sometimes too low to feed the bacteria fast enough, sometimes high enough to stall the cycle. You also end up with a layer of decomposing food and brown muck in the tank, which is unsightly and takes a thorough clean before stocking.

If you choose this method, still test daily with a liquid test kit and watch for the same pattern: ammonia spike, nitrite spike, then both falling to zero with nitrate climbing. The progression is identical; only the timing and precision change.

How to know your tank is genuinely ready

The criteria are unambiguous:

• You dose ammonia to 2 ppm (or a final pinch of food)
• 24 hours later, ammonia reads 0 ppm
• Nitrite reads 0 ppm
• Nitrate reads above 0 ppm (typically 10–40 ppm)
• You see the same result two days in a row

If even one of these fails, the cycle is not complete. Do not add fish yet. Adding fish to a partially-cycled tank exposes them to the same toxic conditions you've spent weeks trying to prevent.

The day you add fish: final preparation

Before you visit the fish shop:

Do a large water change. Right at the end of a cycle, your nitrate levels are usually high — sometimes 40–80 ppm — because nothing has been removing it. A 50–75% water change brings nitrate down to a safe baseline before fish arrive. Use dechlorinated water at the same temperature as the tank.

Do not skip the dechlorinator. Australian municipal water contains chlorine or chloramine, both of which kill the bacteria you spent weeks growing. Always treat fresh tap water with a conditioner like Seachem Prime or API Tap Water Conditioner before it touches your tank.

Stock gradually, not all at once. Your bacteria population is sized for the ammonia load you've been dosing. Adding twenty fish on day one creates more waste than the colony can process, triggering a mini-cycle and ammonia spike. Add a small group first (no more than a third of your intended stock), then wait two weeks before adding more. Test ammonia and nitrite during this period to catch any spike early.

Common problems and how to read them

Nothing is happening after two weeks

Either your ammonia dose is too high (above 4 ppm can stall the cycle), your pH is too low (below 6.5 slows bacterial activity dramatically), or your temperature is too cool (bacteria establish fastest between 28 and 30°C). Test your pH and temperature, and reduce ammonia dosing if needed.

Ammonia drops but nitrite is stuck high for weeks

This is normal. The nitrite-converting bacteria are slower to establish than the ammonia-converters, and the nitrite spike is the longest stage of the cycle. Keep going — it will resolve, often suddenly, somewhere between weeks three and five.

My pH crashed

The cycling process produces nitric acid as a byproduct, which depletes the buffering capacity of the water over time. If pH drops below 6.5, the cycle stalls. A small water change (25%) brings the pH back up and restores buffering. Test pH weekly and water-change if it starts to drop.

The cycle "stalled" — readings have stopped progressing

Usually one of: pH too low (fix with a water change), too much ammonia (let it drop, then dose lower), or the bacteria are starving (you stopped dosing too soon). Diagnose by checking pH and current readings, then adjust.

How to keep a cycled tank cycled

Once cycling is complete and fish are in, the bacterial colony self-regulates to match your stocking and feeding levels. Three things will damage or destroy that colony:

Cleaning filter media in tap water. Chlorine kills bacteria. Rinse media only in old tank water (the water removed during a water change), and only when flow is genuinely restricted — not on a routine.

Replacing all filter media at once. Most of your bacteria live in the filter. Swapping all media simultaneously throws out the colony. Replace media gradually if it physically breaks down, and only one component at a time.

Long power outages or extended dry periods. Without water flow, the bacteria suffocate within 24–48 hours. If your filter has been off for more than a few hours, expect a mini-cycle and test daily for ammonia for the following week.

What to do next

Once you're confident on the theory, the practical sequence is straightforward:

1. Set up your tank fully — substrate, filter, heater, dechlorinated water, lights on a timer.
2. Choose your cycling method — pure ammonia for precision, fish food if you can't source ammonia.
3. Start dosing, test daily, and record every reading.
4. Be patient through the nitrite spike. It's normal and resolves.
5. Confirm a full 24-hour process of 2 ppm ammonia to zero ammonia and zero nitrite, two days running.
6. Do a large water change to lower nitrate.
7. Add your first small group of fish.
8. Test ammonia and nitrite daily for the first fortnight after adding fish.

A patient cycle is the single best gift you can give your future fish. The four to six weeks of waiting feels long, but it sets up years of healthy, low-maintenance fishkeeping. Rushing this step is the most common reason new tanks fail.

This guide reflects modern fishless cycling practice. For specific dosing instructions, always follow the chart on your chosen ammonia or bacterial starter product. If you'd like to suggest a correction or improvement, the source for this article is reviewed periodically.