How to cycle a betta tank
Three to six weeks of dosing ammonia and testing, before the fish. It is the one step that separates the tanks that work from the ones that lose a fish in month one.
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What cycling actually is
Your fish produces ammonia. Ammonia is lethal well below the concentration you could detect by eye. Two families of bacteria fix this: one converts ammonia to nitrite (also lethal), the other converts nitrite to nitrate (largely harmless at the levels a betta tank reaches, and removed by water changes and plants).
Those bacteria colonize your filter media over three to six weeks. Cycling is growing them before there is a fish depending on them.
There is no product that removes the wait. Bottled bacteria shortens it - Seachem publishes a seven-day dosing schedule for Stability, not a one-day one, which tells you how they see it themselves.
What you need
- A liquid test kit for ammonia, nitrite and nitrate. Not strips - during a cycle you need to distinguish 0 from 0.25 ppm and strips cannot resolve that.
- An ammonia source. Pure ammonium chloride solution sold for fishkeeping, or household ammonia with no surfactants, perfumes or detergents. If it foams when shaken, do not use it.
- Bottled nitrifying bacteria. Optional but genuinely useful.
- Dechlorinator.
- The tank running - filter and heater on. Bacteria colonize faster at 78°F than at 68, so the heater matters during the cycle too.
The procedure
- 01
Set the tank up completely and run it
Substrate, hardscape, plants, filter and heater all in and running. Plants tolerate the ammonia involved perfectly well, they establish during the wait, and a planted tank often cycles faster because plants consume ammonia directly. The setup order.
- 02
Dose ammonia to 2 ppm
Add ammonia, test, and adjust until you read about 2 ppm. Note how much it took - that is your dose for the rest of the cycle. Two ppm is deliberately more than a betta produces; you are building a colony with headroom.
- 03
Add bottled bacteria on the published schedule
Seachem publishes Stability at one capful (5 mL) per 10 US gallons on day one, then one capful per 20 US gallons daily for seven days. For a rated 5-gallon holding 4.3 gallons that is 2.15 mL on day one and 1.08 mL daily after. Shake the bottle, and turn off any UV.
- 04
Test daily and write it down
Ammonia and nitrite every day, nitrate every few days. A written record is what lets you see the cycle progressing; a single reading tells you almost nothing. Expect ammonia to hold, then start falling as nitrite appears.
- 05
Re-dose ammonia whenever it hits zero
The colony needs feeding continuously. Every time ammonia reads zero, dose back to 2 ppm. Letting it sit at zero for days starves the bacteria you are trying to grow, which is the most common way a cycle stalls.
- 06
Wait for nitrite to spike and then fall
Nitrite typically appears in week two and climbs high - often off the top of the test chart. That is normal and it is the sign the first bacterial family is working. The second family takes longer to establish, so nitrite usually stays high for a week or more before dropping.
- 07
The test that ends it
When a full 2 ppm ammonia dose reads zero ammonia and zero nitrite within 24 hours, and you have nitrate present, the tank is cycled. Do a large water change - 50 to 80% - to bring the accumulated nitrate down, and add the fish.
The timeline, roughly
| Week | Ammonia | Nitrite | Nitrate |
|---|---|---|---|
| 1 | Holds around 2 ppm | 0 | 0 |
| 2 | Starting to fall | Appearing and climbing | 0 or trace |
| 3-4 | Falling to 0 in a day | High, then falling | Rising |
| 4-6 | 0 within 24h of dosing | 0 within 24h | Present - do a big change |
Wide variation is normal. Temperature, pH, whether you seeded with established media and how heavily planted the tank is all move it. Six weeks is not a failure.
The fastest legitimate shortcut
A handful of squeezed filter media, or a used sponge, from an established healthy tank. That is a transplanted colony rather than a bottled one and it can cut the cycle to under two weeks.
Ask a local shop or a friend. Keep it wet and in tank water in transit - media that dries out is media that has died.
If the fish is already in the tank
It happens. Here is the honest version rather than a lecture.
- Do not dose ammonia. The fish is supplying it.
- Test daily, without exception. Ammonia and nitrite.
- Water change whenever ammonia or nitrite is detectable at all. 25-50%, temperature-matched, dechlorinated. Possibly every day for weeks.
- Dose Prime. Seachem publishes that it binds ammonia, nitrite and nitrate, which buys the fish time between changes. Emergency dosing of up to five times the normal amount for nitrite is stated on their directions.
- Feed very lightly. Every uneaten granule is more ammonia. A betta will not suffer from a couple of fasting days a week.
- Add bottled bacteria on the published schedule.
It works, it is far more effort than a fishless cycle, and the fish carries the risk of every mistake. That is the whole argument for doing it in the other order.
Why the cycle stalls, in order of likelihood
- Ammonia went to zero and stayed there. The colony starved. Re-dose.
- Media was rinsed under the tap. Chlorine killed it. Start again.
- Ammonia was dosed far too high. Above about 5 ppm inhibits the bacteria you are cultivating. Water change down to 2.
- The tank is cold. Bacteria colonize slowly below about 70°F. Check the heater is actually running.
- You are testing with strips. They cannot resolve the range that matters and will tell you nothing useful about the transition.
What we would buy for this
Chosen from the same registry as our roundups, on published specifications and the arithmetic they imply - not on testing we have not done.
Seachem Stability
Bottled nitrifying bacteria with a published seven-day schedule, which is what makes it useful rather than mysterious. Seachem publishes the exact dose: one capful per 10 gallons on day one, then one capful per 20 gallons daily for a week. In a 5-gallon that is 2.5 mL then 1.25 mL - a bottle lasts a very long time.
Skip it if: You expect it to replace cycling. It seeds bacteria; it does not conjure a biofilter overnight, and Seachem does not claim it does.
Seachem Prime
The dechlorinator whose published dosing makes it the obvious nano choice: 5 mL treats 50 US gallons, so a 5-gallon water change needs half a milliliter. A 100 mL bottle is therefore roughly two hundred water changes. It also binds ammonia and nitrite, which is the property that turns a cycling accident into a survivable week.
Skip it if: You need something you can dose by the capful into a nano. At this concentration a capful in a 5-gallon is a ten-times overdose - you are measuring in drops.
Nano sponge filter with air pump
The cheapest correct answer for a betta, and the one we would fit if we could only fit one thing. A sponge filter has no impeller, no outflow jet and no way to shred a fin. It is also the only filtration in this list that keeps working when the power comes back on after an outage without needing to re-prime. What it does not have is a manufacturer: these are sold under a dozen interchangeable brands with no published specifications at all, and we are not going to pretend otherwise.
Skip it if: You cannot live with an air pump humming on your desk, or with a black sponge in the corner of the display.
Frequently asked
How long does it take to cycle a betta tank?
Three to six weeks fishless. Faster with seeded media from an established tank - sometimes under two weeks - and slower in a cold tank or one where ammonia was allowed to run out.
Can I cycle a tank with the fish in it?
You can, and it means daily testing and frequent water changes for several weeks, with the fish carrying the risk. The procedure is in the section above. Fishless is better in every respect except that it requires patience.
Do bottled bacteria actually work?
They help. Seachem publishes a seven-day schedule for Stability rather than claiming instant results, which is a reasonable indication of what to expect. Squeezed media from an established tank works considerably better.
How do I know when my tank is cycled?
A full 2 ppm ammonia dose processes to zero ammonia and zero nitrite within 24 hours, with nitrate present. Nothing else counts - not elapsed time, not clear water, not a bottle of bacteria.
Should I do water changes while cycling?
In a fishless cycle, no - only at the end, and only if nitrate is very high. Water changes during a fishless cycle dilute the ammonia you are deliberately supplying. With a fish in the tank, the opposite: change water whenever ammonia or nitrite is detectable.