Cycle Your Water — The Fun, Fail-Safe Way
Definitive Water Cycling Guide for Experienced Keepers
Growing friendly bacteria turns fish pee/poo (ammonia) into safer stuff. Skip it and you risk New Tank Syndrome. We’ll keep it simple, visual, and forgiving.¹
Establish robust nitrification to prevent NH3/NO2− toxicity and avoid NTS. Includes validated fishless protocol, fish-in mitigation, chloramine handling, and troubleshooting.¹²
🌈 What is Cycling & Why Does it Matter?
Think of your aquarium like a brand new town. At first, there’s no rubbish system in place… if people moved in (your fish 🐟), the streets would quickly fill with mess! Cycling is how we build the waste-cleaning system before any residents arrive. 🚮➡️🌱
🚫 Without Cycling
- Fish get sick or die 😢
- Ammonia builds up like poison
- Your tank turns smelly and cloudy
✅ With Cycling
- Healthy, happy fish 🐠
- Stable, clean water 🌊
- A thriving mini-ecosystem 🌿
💡 Tip: Cycling is like giving your fish a safe home before they move in. Don’t skip it!
Why Cycling is Essential
The nitrogen cycle is the biochemical foundation of all closed aquatic systems. Without a functioning microbial community, organic waste is rapidly converted to toxic ammonia (NH₃/NH₄⁺), leading to fish stress, disease susceptibility, and mortality. Establishing nitrifying bacteria before stocking ensures predictable water chemistry and reduces reliance on chemical binders.
Risks of Skipping Cycle
Ammonia spikes, nitrite toxicity, opportunistic infections, and destabilised pH buffering.
Benefits of Proper Cycle
Stable nitrogen processing, reduced mortality, optimal conditions for biofilm growth, and long-term tank sustainability.
🪄 Your Simple Cycling Journey
Follow these steps to grow your tank’s friendly bacteria army 🦠. Think of it like raising tiny invisible gardeners who’ll keep your water safe!
Set Up Your Tank
Add filter, heater, substrate, and decorations — but no fish yet! 🎣
💡 Tip: Plants help! They give bacteria extra surfaces to live on.Add an Ammonia Source
A tiny pinch of fish food 🐟🍽️ every day or a few drops of bottled ammonia. This “feeds” the bacteria.
Test the Water
Use a liquid test kit 🧪. First you’ll see ammonia, then nitrite, and finally nitrate.
Wait & Watch
It usually takes 4–6 weeks. When ammonia AND nitrite hit 0, your bacteria army is ready. 🎉
💡 Tip: Don’t rush — cycling is like baking a cake, it needs time in the oven.Step-by-Step: Establishing the Biofilter
Use a fishless cycling protocol for full control and reproducibility. Below is the validated 4-stage approach:
System Preparation
Install filtration, heater, and substrate. Ensure full circulation and dechlorinate with Prime®/Safe™ to neutralise chlorine/chloramine.
Initial Ammonia Dose
Target 2–3 ppm NH₃-N using pure ammonium chloride or household ammonia (no surfactants). Record baseline parameters.
Monitoring & Redosing
Test daily for NH₃/NH₄⁺ and NO₂⁻. Once NH₃ drops near zero within 24–48h, redose to maintain ~2 ppm. Expect NO₂⁻ accumulation before NO₃⁻ appears.
Cycle Completion
Confirmed when a full 2 ppm NH₃-N dose is metabolised to NO₃⁻ within 24h, and NO₂⁻ simultaneously returns to zero. Perform ≥80% water change to reduce nitrate before stocking.
🔄 Meet the Nitrogen Cycle!
This is your tank’s clean-up crew. Tiny, invisible bacteria take fish waste and leftovers and turn them into safe stuff for your fish. 🦠➡️🌿
💡 Tip: Don’t worry if it feels complicated – just remember: bad stuff ➡️ less bad stuff ➡️ safe stuff.
The Nitrogen Cycle Explained
Aquatic nitrification follows a two-stage process mediated by autotrophic bacteria:
- Ammonia Oxidation Ammonia (NH₃/NH₄⁺) → Nitrite (NO₂⁻) *Carried out primarily by Nitrosomonas spp.*
- Nitrite Oxidation Nitrite (NO₂⁻) → Nitrate (NO₃⁻) *Carried out by Nitrospira spp. (formerly attributed to Nitrobacter).*
Accumulated nitrates are removed through water changes, plant uptake, or denitrification in anaerobic zones.
🐟 What If I Already Added Fish?
Don’t panic! Many people add fish too early. This is called Fish-In Cycling. It just means the bacteria are still growing while fish are living in the tank.
Test Often
Check water every 1–2 days. Watch for ammonia and nitrite.
Water Changes
Do small water changes (25–50%) if levels go up. Fresh water = safer fish!
Use a Helper
Add a conditioner like Seachem Prime® — it helps protect fish while bacteria catch up.
Be Patient
It takes longer, but you’ll get there. Your fish will thank you for the extra care. ❤️
Fish-In Cycling (Emergency Protocol)
If livestock are present before nitrifiers are established, the goal is to mitigate acute NH₃/NO₂⁻ toxicity while still seeding the biofilter.
Frequent Testing
Measure NH₃/NH₄⁺ and NO₂⁻ at least daily. Keep NH₃ < 0.25 ppm and NO₂⁻ < 0.25 ppm (with Prime® binding if needed).
Aggressive Water Changes
Perform 25–50% changes whenever thresholds are exceeded. Always dechlorinate to preserve nascent biofilm.
Detoxify & Support
Dose Prime®/Safe™ every 24–48h at up to 5× label concentration for binding. Supplement with bottled nitrifiers (Dr. Tim’s One & Only, Tetra SafeStart).
Feeding Control
Restrict feeding to limit nitrogen input. Better slightly underfed than exposed to chronic NH₃/NO₂⁻ stress.
Note: Fish-in cycling prolongs the establishment phase and carries elevated risk. It should be avoided if possible, but can be stabilised with disciplined husbandry and chemical support.
🧪 Testing Made Easy
Testing your water is like a check-up for your fish tank. You’ll see the cycle happen step by step!
| Stage | What You’ll See | What It Means |
|---|---|---|
| Start | Ammonia goes up 📈 | Fish food/pee turns into waste |
| Middle | Nitrite appears 🧪 | Bacteria #1 are working |
| Later | Nitrate shows up 🌿 | Bacteria #2 are working |
| Finish | Ammonia + Nitrite = 0 🎉 | Your tank is safe for fish |
Parameter Targets for Validation
Use a liquid drop test kit (API Freshwater Master, JBL, or equivalent) for quantitative results.
| Phase | NH₃ / NH₄⁺ | NO₂⁻ | NO₃⁻ | Action Point |
|---|---|---|---|---|
| Initiation | 2–3 ppm | 0 ppm | 0 ppm | Dose ammonia, record baseline |
| Nitrite Rise | ↓ | ≥1 ppm | Trace | Continue redosing to ~2 ppm |
| Nitrate Accumulation | Near 0 | High | ≥40 ppm | Prepare for final validation |
| Completion | 0 ppm | 0 ppm | Present | Perform ≥80% water change, stock |
Validation rule: System is cycled when a 2 ppm NH₃-N dose is fully oxidised to NO₃⁻ within 24h with no residual NH₃/NO₂⁻.
🧫 Bottled Bacteria: The Helpful Shortcuts
These products add the “good germs” that make your water safe. They don’t replace patience, but they can make cycling faster and smoother.
Nitrico Bacterial Goop
Live nitrifying bacteria in a pouch. Helps stop ammonia & nitrite spikes in new tanks or when adding fish.
- Refrigerate if label says to ❄️
- Turn off UV/ozone for 48 hrs
- Don’t rinse filter media in tap water
Dr. Tim’s One & Only
Trusted live nitrifiers designed for fishless cycling. Often paired with ammonium chloride for precise dosing.
- Target ~2 ppm ammonia
- Test daily for progress
- Big water change before stocking
API Quick Start / Tetra SafeStart
Adds beneficial bacteria to help kick-off cycling or support after water/filter changes.
- Shake well before use
- Dose after dechlorinator
- Keep within expiry dates
Dechlorinator
Always treat new water first — chlorine/chloramine can hurt your good bacteria.
Temperature
Warm (24–28 °C) helps bacteria grow faster during cycling.
Gentle Flow
Steady filter flow = oxygen for bacteria. Don’t turn filters off.
Live Nitrifiers & Chemical Supports
Use bottled autotrophic nitrifiers to seed the biofilter; pair with binders to mitigate acute toxicity during fish-in events.
Nitrico Bacterial Goop
UK-available live nitrifying consortium intended to suppress NH₃/NO₂⁻ excursions in immature filters and during stock additions.
- Disable UV/ozone for 48 h post-dose
- Keep within storage temperature guidance
- Feed with measured NH₃-N (fishless) or minimal diet (fish-in)
Dr. Tim’s One & Only
Designed for fishless cycling with paired ammonium chloride. Validate when 2 ppm NH₃-N → 0 NH₃/NO₂⁻ within 24 h.
- Dose to 2–3 ppm NH₃-N
- Redose as NH₃ hits ~0
- ≥80% water change before stocking
API Quick Start / Tetra SafeStart
Commercial nitrifier blends; dose after dechlorination and avoid biocides/UV for 24–48 h. Monitor NO₂⁻ transients.
- Use fresh, in-date bottles
- Maintain oxygenation
- Do not rinse media in chlorinated water
Binders (Prime®/Safe™)
Use per label; in emergencies, up to 5× dose to detoxify NH₃/NO₂⁻. Re-dose every 24–48 h while testing.
Interactions
Dechlorinate first; dose bacteria after. Pause UV/ozone; maintain strong aeration for obligate aerobes.
Validation
Biofilter considered operational when 2 ppm NH₃-N is oxidised to NO₃⁻ with 0 NH₃/NO₂⁻ at 24 h.
📅 The Cycle Journey at a Glance
Think of cycling like a storybook: your tank grows its tiny cleaning crew step by step. Here’s the simple version:
Start ➡️ Ammonia
Fish food/pee makes ammonia appear.
Bacteria #1 ➡️ Nitrite
First bacteria eat ammonia, but leave nitrite (still harmful).
Bacteria #2 ➡️ Nitrate
Second bacteria eat nitrite, leaving nitrate (safer but still needs water changes).
Finish ➡️ Safe Tank!
Ammonia + Nitrite = 0. 🎉 Fish can safely move in!
Cycle Timeline (Approximate)
Duration varies with temperature, KH, seeding, and bacterial product used. Below is a generalised progression for fishless cycling:
Day 0 Ammonia Dosed
2–3 ppm NH₃-N dosed. No nitrite/nitrate detected.
Days 3–14 Nitrite Spike
NH₃ declines, NO₂⁻ rises significantly. Maintain redose to ~2 ppm as NH₃ drops.
Weeks 3–6 Nitrate Accumulation
NO₂⁻ begins to fall, NO₃⁻ climbs (>40 ppm). System approaching completion.
Week 4–8 Cycle Complete
2 ppm NH₃-N is oxidised to NO₃⁻ within 24 h, with 0 NH₃/NO₂⁻. Perform ≥80% water change, stock gradually.
🙈 Common Mistakes to Avoid
Don’t worry — everyone makes mistakes when starting out! Here are the top “oopsies” so you don’t fall into the same traps:
🚀 Adding Fish Too Soon
Your tank needs time first — rushing = sick fish.
🚱 Not Using Dechlorinator
Tap water kills good bacteria unless treated.
🧽 Cleaning Too Much
Don’t scrub filters in tap water — you wash away the bacteria army!
🍔 Overfeeding
Too much food = too much waste = scary ammonia spikes.
Critical Pitfalls in Cycling
Even experienced keepers occasionally compromise cycle stability. Avoid these advanced pitfalls:
Oversterilisation
Excessive filter/media rinsing in chlorinated water destroys nitrifiers and prolongs establishment.
Chloramine Oversight
Failure to neutralise chloramine yields persistent NH₃ release, destabilising validation results.
Misinterpreting Test Kits
API liquid kit NO₂⁻ maxes at 5 ppm — a “plateau” can be misread as stability. Dilute samples if needed.
Instant-Cycle Assumptions
Seeded media/bottled bacteria reduce time, but do not bypass the need for full validation (2 ppm → 0 NH₃/NO₂⁻ within 24 h).
Note: Patience and validation are essential even when using seeding or bacterial inoculants.
✅ Quick Recap — You’ve Got This!
Run through this checklist before you add fish. If everything’s ticked, you’re golden. 🥇
- Tank set up: filter, heater, decor, plants (optional)
- Dechlorinator used on all new water
- Ammonia source added (food pinches or bottled)
- Testing regularly with a liquid kit (not strips)
- Ammonia reads 0
- Nitrite reads 0
- Nitrate present (do a big water change)
- Big water change done (50–80%)
- Stock slowly and keep testing for a week
Validation Checklist (Fishless Cycle)
Use this before first stocking or after major disturbances.
- System prepped: stable temp (24–28 °C), oxygenation, flow across media
- Dechlorinated for chlorine/chloramine; no residual biocides/UV
- Dosed to ~2 ppm NH₃-N and recorded baseline
- 24 h oxidation test: 2 ppm → 0 NH₃ / 0 NO₂⁻
- Nitrate accumulated (document value)
- ≥80% water change completed to reset NO₃⁻
- Gradual stocking plan; verify zero NH₃/NO₂⁻ next day
- Contingency: Prime®/Safe™ on hand for binding if needed
- Log kept (dates, doses, readings) for traceability
🛠️ Troubleshooting — Don’t Panic!
If something looks weird, use this guide. Check the problem that matches what you see and follow the steps.
What you’ll see
- Ammonia stays high for days
- No/low nitrite
Likely causes
- Too little “good bacteria” started
- Dechlorinator not used (chlorine hurt bacteria)
- Filter turned off or weak flow
Fix it
- Always treat tap water with dechlorinator
- Add bottled bacteria (shake well, turn off UV for 24–48h)
- Check filter is on 24/7 and water is moving
- Keep temp ~24–28 °C
What you’ll see
- Nitrite stays purple for ages
- Nitrate is rising
Likely causes
- Stage-2 bacteria are still growing
- Feeding too much ammonia/food
Fix it
- Be patient — this stage can take longest
- Stop adding extra food/ammonia for a few days
- Water change if fish are in (25–50%)
What you’ll see
- Orange/red nitrate test
Likely causes
- Cycle worked! But nitrate built up
Fix it
- Do a big water change (50–80%)
- Keep some plants — they help use nitrate
What you’ll see
- Milky/hazy water
Likely causes
- Bacteria bloom (normal)
- Too much food
Fix it
- Wait a few days — it often clears on its own
- Feed less and keep the filter running
What you’ll see
- pH falling low, fish look stressed
Likely causes
- Very low KH (buffer)
Fix it
- Water change to restore minerals
- Use a buffer/crushed coral if your water is soft
🚑 Fish already in the tank?
Do this if fish are breathing fast, gasping, or you see high ammonia/nitrite:
- Change 25–50% water now (treat with dechlorinator)
- Dose conditioner (e.g., Prime®)
- Feed tiny amounts for a while
- Test daily until stable
Diagnostics & Remedies
Match the presentation, check the likely failure mode, then apply the targeted fix.
Presentation
- NH₃ remains elevated; NO₂⁻ minimal
Failure modes
- Insufficient nitrifier biomass / dead inoculant
- Residual disinfectants (chlorine/chloramine)
- Insufficient O₂/flow (filter off, clogged media)
Remedies
- Re-inoculate (fresh, in-date live cultures; pause UV/ozone 24–48 h)
- Verify dechlorination; for chloramine, treat full tank volume
- Increase aeration and ensure constant circulation through media
Presentation
- NO₂⁻ maxed at kit ceiling; NH₃ near 0; NO₃⁻ rising
Failure modes
- Nitrite oxidisers (Nitrospira) lagging
- Overdosing NH₃ causing inhibitory NO₂⁻ peak
- Test saturation (API tops at ~5 ppm)
Remedies
- Hold NH₃ dosing; allow NO₂⁻ to fall
- Dilute test sample (1:1 or 1:4) to read beyond ceiling
- Seed media or add live nitrifiers
Presentation
- NO₃⁻ high; NH₃/NO₂⁻ 0 on 24 h test
Interpretation
- Cycle complete; nitrate accumulation typical
Remedies
- ≥80% water change before first stock to reset NO₃⁻
Presentation
- Apparent NH₃ reading after water change
Failure modes
- Monochloramine release → transient ammonia reading
- Insufficient dechlorination dose/hold time
Remedies
- Use full-volume dechlorination; allow mixing time
- Re-test after 1–2 h; confirm with 24 h oxidation test
Presentation
- pH drifting down; nitrification sluggish
Failure modes
- KH depleted by nitrification acids; soft source water
Remedies
- Large water change; restore alkalinity
- Add buffering media (crushed coral/alkalinity buffer)
Presentation
- Unexpected “0” NH₃/NO₂⁻ with ongoing symptoms
Failure modes
- Reagent age, improper shake/timing
- Nitrite interference in nitrate test (incomplete step 1)
- Prime®/binders affecting perception (still testable, but interpret trends)
Remedies
- Use fresh, in-date kits; follow timing precisely
- For nitrate, ensure adequate shake time and full step sequence
- Trend multiple days; confirm with alternate kit if in doubt
Emergency: Fish-In Toxicity
- Immediate 25–50% WC (dechlorinate full volume)
- Dose Prime®/Safe™ (per label; up to 5× in emergencies)
- Maximise aeration; reduce feeding
- Re-test in 24 h and repeat as needed
❓ Beginner FAQs
Still confused? Don’t worry — here are the most common beginner questions answered simply:
How long does cycling take?
Usually 4–6 weeks. Using bottled bacteria or seeded media can shorten it.
Can I add fish straight away?
No. Adding fish before cycling risks “New Tank Syndrome.” Wait until ammonia and nitrite both read 0.
What’s ammonia?
It’s the waste from fish pee/poo and leftover food. In high amounts it’s poisonous.
What test kit should I use?
A liquid drop kit (like API Freshwater Master). Strips are less accurate.
Why do I need to feed bacteria?
They need “food” (ammonia) to grow. Without it, they won’t multiply.
What if my water goes cloudy?
That’s a bacteria bloom — normal! It clears as the cycle balances.
Do plants help with cycling?
Yes! They use ammonia/nitrite/nitrate and provide surfaces for bacteria.
Can I speed it up?
Yes, by using bottled bacteria, live plants, or squeezing filter gunk from an established tank.
What’s nitrate?
The final, safer waste. Still needs water changes to keep levels low.
What happens if I skip cycling?
Fish get stressed, sick, or die. Cycling is what makes a safe home for them.
❓ Experienced FAQs
Advanced keeper questions — detailed answers for full control:
What’s the validated endpoint of a fishless cycle?
Ability to oxidise 2 ppm NH₃-N to 0 NH₃/NO₂⁻ within 24 h, with nitrate present.
Can I cycle with chloramine-treated tap water?
Yes, but you must use a dechlorinator that handles chloramine, as it breaks into chlorine + ammonia.
Do binders like Prime® interfere with test results?
They detoxify but don’t remove ammonia/nitrite, so tests still show them. Interpret trends, not absolute values during binding use.
What’s the optimal KH for nitrification?
≥4 dKH (~70 ppm). Lower KH can lead to acidification and cycle stall.
How does temperature affect cycle speed?
Optimum 26–30 °C. Below ~20 °C, nitrification slows significantly.
Can I instantly cycle with seeded media?
Seeding reduces time dramatically, but always validate with the 24 h 2 ppm oxidation test.
Why did my NO₂⁻ plateau at max kit reading?
API kit tops out at ~5 ppm. Dilute sample to check real level. Plateau means nitrite oxidisers lagging.
What’s the risk of overdosing ammonia?
>5 ppm can inhibit nitrite oxidisers and prolong the cycle.
Can UV sterilizers run during cycling?
No — they kill free-floating nitrifiers. Keep off for at least 48 h after dosing bottled bacteria.
How often should I re-check after stocking?
Daily for the first week post-cycle, then weekly. Always after large changes or new additions.