How to cycle a fish tank

Cycling means growing two populations of bacteria in your filter before any fish go in — one that converts ammonia to nitrite, and one that converts nitrite to nitrate. You do it by adding an ammonia source to an empty, running tank and testing until the tank clears a full ammonia dose to 0 ppm ammonia and 0 ppm nitrite within 24 hours. That takes four to six weeks without a bacterial starter, and often two to three with one.

If you read one page on this site, this is it. Everything else — which filter, which heater, which fish — is downstream of whether this step happened.

What is actually happening

Fish excrete ammonia continuously, through their gills as well as in their waste. Uneaten food and decaying plant matter add more. Ammonia is acutely toxic: it burns gill tissue at concentrations well under 1 ppm, and — as the EPA documents — its toxicity rises with both pH and temperature. A tropical community tank is warm by definition, which is the worst case.

In an established aquarium none of that matters, because two groups of bacteria living in the filter media handle it in sequence. This is ordinary nitrification, the same two-stage bacterial oxidation that happens in soil and in wastewater treatment:

1. Ammonia-oxidizing bacteria convert ammonia (NH3/NH4+) into nitrite (NO2-). Nitrite is also toxic — it interferes with the blood's ability to carry oxygen — so this alone does not make the water safe.
2. Nitrite-oxidizing bacteria convert nitrite into nitrate (NO3-). Nitrate is far less toxic and accumulates slowly, which is what your weekly water change exists to remove.

Those bacteria are not in the box. They are not in the water, not in the gravel, and not in a new filter cartridge. They arrive from the air and from whatever you add, and they multiply slowly — this is a biological population growing, not a chemical reaching equilibrium, which is why it cannot be rushed by effort.

What you need before you start

• A liquid test kit that reads ammonia. Non-negotiable. Cycling is a process you can only see through test results — the water looks identical throughout. The common six-in-one strips do not test ammonia at all: see test strips vs liquid test kits.
• An ammonia source. Pure ammonium chloride solution is the cleanest option. Fish food left to decay also works and is free, but the dose is unmeasurable and it fouls the water.
• Dechlorinator, because chlorine and chloramine kill the bacteria you are trying to grow just as effectively as they kill fish.
• The tank fully set up and running — filter, heater, substrate, decor. Bacteria colonize surfaces, so everything should be in place from the start.
• Optionally, a bacterial starter, which is the one thing that genuinely shortens this. See do bacteria starters work.
• Patience. The only ingredient that cannot be bought.

The fishless cycle, step by step

1. Set the tank up completely and run it for 24 hours. Filter, heater at your target temperature, substrate, decor. Check for leaks and confirm the heater holds. Warmth matters — bacteria multiply faster at tropical temperatures, so a properly heated tank cycles faster than a cold one.
2. Dose ammonia to about 2 ppm and test to confirm you actually hit that. Dose rates differ between products, so follow the bottle rather than a figure from a web page. Two parts per million is enough to feed a colony sized for a stocked tank without being so high it inhibits the bacteria.
3. Add a bacterial starter, or better, seed material from an established tank. A squeezed-out filter sponge, a handful of gravel or a piece of media from a healthy running tank is the fastest method there is, because it moves an existing colony rather than growing one.
4. Test ammonia and nitrite every day or two, and write the numbers down. The written record is what tells you the cycle is progressing; a single reading tells you almost nothing.
5. Wait for nitrite to appear. Ammonia falls and nitrite rises, usually somewhere in the first one to two weeks. This is the first population establishing, and it is the first real evidence anything is happening.
6. Re-dose ammonia back to 2 ppm whenever it reads zero. The colony is sized by how much it is fed. Stop feeding it and it starves back down, and you lose progress.
7. Wait for nitrite to fall and nitrate to appear. This is the slow part, and the second population is genuinely slower to establish than the first. Nitrite can sit high for a week or more before it moves.
8. Test the finish. Dose ammonia to 2 ppm and test 24 hours later. If both ammonia and nitrite read 0 ppm, the tank is cycled. If either shows anything, it is not — re-dose and wait.
9. Do a large water change, 50% or more, to bring the accumulated nitrate down before the fish arrive.
10. Add a few small fish, not the full stocking. The colony is sized to the load it has been fed, and a large addition outruns it. See adding fish to a new tank.

What the numbers do over the month

Things that do not speed it up

• Water changes. They dilute the ammonia you are deliberately feeding the colony. During a fishless cycle they set you back rather than helping.
• Adding more fish. More ammonia than the colony can process is not encouragement, it is an overload.
• Waiting for the water to look clear. Clarity and ammonia are unrelated. A crystal-clear tank can be lethal and a cloudy one can be perfectly cycled — see why is my fish tank water cloudy.
• Extra filtration. More flow does not create bacteria faster; it gives them somewhere to live once they exist.
• Ammonia-removing chemical media. It strips out the ammonia you are using as food. Actively counterproductive here.
• Testing more often. It will not go faster because you are watching. Every two days is plenty.

If fish are already in the tank

Very common, usually a few days after a first trip to the shop, and entirely recoverable. You are now doing a fish-in cycle: the same process, with the fish as the ammonia source and your water changes keeping them alive through it.

1. Test ammonia and nitrite daily. You are managing a number now, not watching a tank.
2. Change 25 to 50% of the water whenever either is detectable, dechlorinated and temperature-matched. This is the entire treatment.
3. Feed sparingly — every uneaten flake becomes ammonia, and fish tolerate light feeding far better than they tolerate ammonia.
4. Use a conditioner that detoxifies ammonia to buy time between changes. It converts ammonia to a less toxic form the bacteria can still consume; it does not remove it and it is not a substitute for the water change.
5. Add bacteria — bottled, or media from an established tank.
6. Do not add any more fish until it is finished.

It takes longer than a fishless cycle because your water changes necessarily keep the ammonia level low, which means feeding the colony less. Full walkthrough on why do my fish keep dying and the fast interventions on how to lower ammonia fast.

After it is cycled

The colony is alive and needs feeding. If you cycle a tank and then leave it empty for a fortnight, the bacteria starve and you are partway back to the start. Add fish within a few days of finishing, or keep dosing ammonia until you do.

And protect the colony afterwards. It lives in the filter media, so: rinse media in old tank water and never under the tap, never replace all your media at once, and treat the instruction on a cartridge box to replace it monthly as the cycling hazard it is. More on that in best aquarium filters.