Easy Guide to Cycling your Tank
& Preparing for your New Tropical Fish
What Is Tank Cycling?
The Cycling process grows beneficial bacteria that turn toxic fish waste (ammonia from urine) into safer compounds. Without it, new fish (especially after postal delivery) can suffer or die within days.
A cycled tank has zero ammonia and zero nitrite at all times, even when fish are producing waste — essential for every tropical species, from tiny Bettas to giant Arowana.
The Nitrogen Cycle
- made simple
Fish release ammonia →
bacteria convert it to nitrite →
other bacteria convert nitrite to nitrate.
Nitrate is not as toxic and is removed by water changes and plants.
Bacteria need oxygen and stable warmth (76–82 °F / 24–28 °C) for most tropicals).
A bit more detail
Fish (and uneaten food) release ammonia (NH₃) through their gills and waste. Ammonia is highly toxic.
Two groups of invisible bacteria do the work:
Ammonia-Oxidising Bacteria (AOB) – mainly Nitrosomonas species – convert ammonia into nitrite (NO₂⁻).
Nitrite-Oxidising Bacteria (NOB) – mainly Nitrobacter species – convert nitrite into nitrate (NO₃⁻).
Nitrate is far less toxic and is removed by weekly water changes or live plants.
The bacteria are aerobic (need oxygen) and grow on surfaces (filter media, gravel, plants).
They multiply slowly — it takes time for their population to match your fish’s waste load.
pH affects toxicity: ammonia becomes more dangerous above pH 7.5.
Temperature speeds things up: 78–82 °F (25–28 °C) is ideal for both bacteria and most tropical fish like Bettas, Channa and Arowana.
Light has no direct effect, but stable temperature and oxygen do.
Why Cycling Differs for Betta, Channa & Arowana
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Betta: Small fish, low waste. A 10-gallon tank can be cycled with modest filtration. Beginners can succeed in 3–5 weeks.
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Channa (Snakeheads): Heavy predators with big appetites. Even a dwarf species creates far more ammonia than a Betta. You need oversized filtration (4–6× turnover) and a longer, more stable cycle.
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Arowana: Giant surface feeders with massive bio-load. They demand the strongest filtration (6–10× turnover) and a fully mature tank (ideally seeded from an established system).
Bottom line: The science is identical, but the scale changes. A Betta can forgive a slightly under-cycled tank for a short time; Channa or Arowana usually cannot.
How Long does Cycling Take?
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How Long Does Cycling Take? Typical timeline (fishless method):
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Days 1–7: Ammonia rises, then drops as AOB appear.
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Days 7–21: Nitrite peaks and falls as NOB appear.
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Days 21–42: Both ammonia and nitrite stay at 0 ppm for a full week = tank is cycled.
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Factors that speed it up:
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Bottled bacteria products
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Seeding with media from a cycled tank
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Higher temperature (within species-safe range)
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Plenty of oxygen and surface area
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Expect 4–8 weeks for a brand-new tank. Never rush — patience saves fish lives.
Three Proven Cycling Methods
Choose One...
Method A – Classic Fishless
(Recommended for Beginners & All Species)
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Fill tank, add heater, filter and dechlorinator.
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Dose pure ammonia to 2–4 ppm (use ammonium chloride or fishless ammonia).
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Test daily. When ammonia drops to 0, re-dose.
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Continue until ammonia and nitrite hit 0 within 24 hours for 7 days straight.
Method B – Seeded / Instant Cycle
Take filter media, gravel or sponge from an established, disease-free tank and add it to your new filter. This can cut cycling time in half. Perfect when receiving Channa or Arowana by post.
Method C – Bottled Bacteria
Products like Seachem Stability, Fritz Turbo Start or Dr. Tim’s One & Only add live bacteria. Follow bottle instructions and dose ammonia as in Method A. Great for busy keepers.
Avoid: Fish-in cycling unless you are very experienced and using tiny, hardy fish with daily 50 % water changes — it is stressful and risky.
Preparing your tank before Fish Arrival
Do this 4–8 weeks before arrival:
• Select proper tank size
• Add heater and filter and turn on.
• Add substrate, plants and décor
• Tap water contains chlorine, usually at very low levels.
• However, chlorine can be damaging to fish so it should be removed
• Chlorine conditioner should be used per the directions on the pack
• The beneficial bacterial colonies will grow and are the brown gunk
that forms on the filter media in the filter.
• The water may look a bit unpleasant initially but will clear with time
• Test water until ammonia + nitrite = 0 for 7 days
Testing and Monitoring
The tools you must have
Buy an API Master Test Kit (or equivalent liquid test) — it is the gold standard. Test daily during cycling, then weekly once fish are in.
Target readings for a cycled tank:
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Ammonia: 0 ppm
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Nitrite: 0 ppm
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Nitrate: 0–20 ppm (higher means it’s working but do a water change)
Keep a simple log: date, time, temperature, test results. This helps you spot problems early.
Safe Acclimatisation for Fish Arriving by Post
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Let sealed bag rest 15 minutes in dim room.
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Float bag 15–30 minutes.
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Drip tank water slowly (45–90 minutes).
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Gently net fish in.
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Keep lights off for 24 hours, no feeding for 24–48 hours.
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For postal arrivals, you must ensure your tank is fully cycled before arriving. This section and our Care Guides for Betta, Channa and Arowana provide more information
First Week Monitoring
& Optimisation
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Test ammonia, nitrite, nitrate & temperature daily.
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25–50 % water change on day 3–4 if needed and weekly thereafter (50% for heavy feeders like Channa and Arowana)
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Watch behaviour closely.
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Maintain stable temperature and gentle flow. Larger fish create more waste — stay extra vigilant.
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Never clean filter media in tap water — rinse in tank water only to preserve bacteria.
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Add fish gradually — start with 25–50 % of final stock and wait 2–4 weeks between additions.
Top Tips:
Common Mistakes & How to Avoid Them
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Adding fish too early → ammonia/nitrite spike → dead fish.
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Over-cleaning the filter → killing the bacteria colony.
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Using soap or chemicals → wipes out bacteria.
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Ignoring temperature swings → bacteria slow down or die.
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Not testing → you won’t know if the cycle is finished.
Always keep a spare sponge filter running in a cycled tank so you have instant “seed” media ready for emergencies.
Indian almond leaves or tannins added for stress relief (especially helpful for Bettas and newly arrived Channa/Arowana).