Get this wrong and you’ll lose fish within a week. Get it right and your tank will be stable for years. Aquarium cycling is the single most important thing you’ll do before adding any livestock — yet it’s also the subject with the most half-answers and contradictory advice online.
This guide covers the actual science, the two main methods, a realistic week-by-week timeline, the numbers you need to hit, and what to do when things go sideways. We’re writing this for the UK market specifically, because tap water chemistry here matters, and because the livestock you’re planning to keep — whether that’s wild-caught Corydoras, a pair of Datnoids, or a Black Arowana (Osteoglossum ferreirai) — will not forgive a half-cycled tank.
Every aquarium produces ammonia (NH₃). It comes from fish waste, uneaten food, and decaying organic matter. In tap water, the chloramines used by most UK water companies will also release ammonia as they break down — worth knowing before you even fill the tank.
Ammonia is acutely toxic. At pH 7.5 and 25 °C, even 0.5 ppm (mg/L) will cause gill damage in most fish. At pH 8.0+, the toxicity roughly doubles because a greater proportion exists as free ammonia (NH₃) rather than the less toxic ammonium ion (NH₄⁺). This pH dependency is one reason blackwater species — kept at pH 5.5–6.5 — are slightly more forgiving during cycling; the equilibrium shifts toward ammonium.
The nitrogen cycle is the biological process that converts this ammonia into progressively less harmful compounds:
These bacteria — collectively referred to as nitrifying bacteria — are obligate aerobes. They colonise porous surfaces: filter media (bio-media, sponge, ceramic rings), substrate, and even the glass walls. This is why the filter is the most critical piece of hardware in a cycled tank, and why you must never wash filter media under tap water. Chlorine and chloramine will kill your bacterial colony.
Most UK tap water contains chlorine or chloramine. Chlorine off-gasses within 24 hours of standing; chloramine does not — it’s a stable compound and requires a dechlorinator that specifically neutralises it (Seachem Prime is widely used and handles both). Check your water supplier’s annual quality report to confirm which your area uses.
Also note your starting parameters:
Fishless cycling uses an ammonia source without any livestock present. It’s faster, more controllable, and doesn’t expose any animals to toxic water. There is no credible argument for not using this method if you’re setting up a new tank from scratch.
Bacterial additives (Seachem Stability, API Quick Start, Dr Tim’s One and Only): These contain live nitrifying bacteria and genuinely accelerate the process — in Marc’s experience, using a quality bacterial additive alongside pure ammonia source can shave 10–14 days off a cycle. They work best when used at double the stated dose in the first week. They are not a magic shortcut — you still need to test and confirm.
Seeded media: If you can beg, borrow, or purchase established filter media from a trusted source (a healthy, disease-free tank), squeeze it into your new filter. A generous handful of mature biological sponge can cut cycling time to under a week. This is how professional retailers cycle new tanks so quickly — and it’s why buying fish from a shop that also sells its mature media is genuinely useful.
Fish-in cycling means adding livestock before the cycle is complete and managing water quality manually while bacteria establish. It was standard practice before fishless methods were popularised, and it’s still used when you’ve received fish unexpectedly or inherited a tank mid-setup.
If you must use this method:
Fish-in cycling is slower because frequent water changes dilute ammonia, which the bacteria need as a food source. It’s a balancing act and it’s stressful — for the fish and for you.
A note on species choice: If you are cycling with fish present, do not do it with expensive or sensitive species. Do not, for example, introduce a rare wild-caught specimen during this period. A pair of common White Cloud Mountain Minnows or a shoal of Zebra Danios will tolerate the process better than most — but they still find it unpleasant.
| Week | What You’ll See | Action |
|---|---|---|
| Days 1–3 | Ammonia stable or rising slowly; readings ~2 ppm. Water may cloud slightly. | Maintain ammonia at 2 ppm. Add bacterial additive at double dose Day 1. |
| Days 4–7 | Ammonia begins to drop. Nitrite detectable (0.25–1 ppm+). | Re-dose ammonia to 2 ppm. Keep temperature at 26–28 °C. |
| Days 7–14 | Ammonia dropping faster. Nitrite spikes sharply — often 2–5 ppm or higher. | Maintain ammonia supply. Do not panic about nitrite — this is the process working. |
| Days 14–21 | Ammonia consistently 0 ppm within 24 hrs of dosing. Nitrite beginning to drop. Nitrate rising. | Reduce ammonia dose slightly to 1 ppm to confirm bacteria are processing it. |
| Days 21–28 | Both ammonia and nitrite returning 0 ppm. Nitrate climbing steadily. | Perform 50% water change to dilute nitrate. Re-test. If ammonia and nitrite stay at 0 ppm 24 hrs later — you’re cycled. |
| Week 4–6 (without additive, from scratch) | As above but nitrite spike may be smaller or delayed. | Same process, longer patience required. |
Timelines vary with temperature, ammonia source, bacterial additive quality, and filter media surface area. A large external canister filter with 1.5 litres of ceramic rings will cycle faster than an internal box filter with a basic sponge.
Do not guess. A test kit is not optional.
| Parameter | During Cycling | Cycled Tank Target | Notes |
|---|---|---|---|
| Ammonia (NH₃/NH₄⁺) | Dosed to 2 ppm; target 0 ppm within 24 hrs by end of cycle | 0 ppm | API liquid test; API strips are too inaccurate. |
| Nitrite (NO₂⁻) | Will spike; must reach 0 ppm before introducing fish | 0 ppm | Also API liquid test. |
| Nitrate (NO₃⁻) | Rising confirms cycle is progressing | <20 ppm (ideally <10 ppm for sensitive species) | Managed by water changes post-cycle. |
| pH | Monitor for crash | Species-dependent (see care guides) | Sharp drops indicate KH exhaustion. |
| KH (carbonate hardness) | Maintain ≥3 dKH | ≥3 dKH | Buffer pH. Replenish with sodium bicarbonate if required. |
| Temperature | 26–28 °C | Species-dependent | Higher = faster bacterial colonisation. |
Test kit recommendation: The API Freshwater Master Test Kit is the standard recommendation — it covers ammonia, nitrite, nitrate, and pH in a single box, uses liquid reagents (more accurate than strips), and has a reasonable shelf life. The Salifert ammonia test is more sensitive if you want to detect sub-0.1 ppm readings. For KH, the JBL KH Test is reliable and UK-available.
A cycle that hasn’t moved in 10+ days is a problem. Here are the most common causes:
If pH drops below 6.0, nitrifying bacteria activity drops sharply and effectively halts below pH 5.5. This happens when KH is depleted by the acid produced during nitrification. Check KH; if it reads 0–1 dKH, raise it by dissolving ½ teaspoon of sodium bicarbonate (bicarbonate of soda) per 40 litres, then re-test pH before adding more. Raise gradually — a sudden pH swing is as dangerous as the original problem.
Counterintuitively, very high ammonia concentrations inhibit the bacteria you’re trying to grow. If you over-dosed, perform a 50% water change to dilute, then re-dose to 2 ppm precisely.
Bacteria colonise very slowly below 20 °C and essentially stop below 15 °C. UK homes in winter can see tank temperatures drop if heaters are undersized. Confirm your heater can maintain 26 °C even in a cold room — budget heaters often can’t.
Standard dechlorinators (sodium thiosulphate) neutralise chlorine but not the ammonia released when chloramine breaks down. If your water contains chloramine and you used a basic dechlorinator, the additional ammonia release will confuse your readings and the chloramine itself may be suppressing bacterial growth. Switch to Seachem Prime or similar chloramine-specific product.
This is the single most common cause of a ‘crashed’ mature tank masquerading as a stalled cycle. Tap water chlorine/chloramine kills nitrifying bacteria rapidly. If this happened, you need to restart. Use RO water or dechlorinated tank water to rinse filter media — always.
Live bacteria products have limited shelf lives and must be stored as directed (usually refrigerated after opening, or at room temperature but away from heat). An expired or heat-damaged product won’t seed anything. Check the bottle date.
A cycled tank is not an infinite capacity for bioload. Stock gradually — adding a huge number of fish at once will spike ammonia beyond what the current bacterial colony can process. Add 25–33% of your intended final stocking, wait two weeks, test, then add the next tranche.
This matters especially for the species MTF specialises in. A Black Arowana (Osteoglossum ferreirai) at 12–13 inches produces a significant bioload. A Datnoid (Datnioides microlepis) at specimen size needs pristine water chemistry to feed confidently and display correctly. There are no shortcuts with rare, expensive, wild-caught livestock — a mature, stable nitrogen cycle is the foundation everything else is built on.
For species-specific parameter targets — temperature ranges, pH, dGH — read the individual care guides. The Corydoras care guide covers wild-caught CW-code Corydoras that are sensitive to even brief nitrite exposure; the Royal Pleco (L190/L191) care guide details the high-oxygen, strong-flow environments these fish need — water quality you can only guarantee with a properly cycled, well-maintained filter.
Before you start: – [ ] Tank filled, dechlorinated (chloramine-compatible product) – [ ] Heater set to 26–28 °C and confirmed accurate with thermometer – [ ] Filter running (bio-media loaded; no carbon) – [ ] Ammonia source and test kits in hand
During cycling: – [ ] Test ammonia, nitrite, nitrate and pH every 24–48 hours – [ ] Maintain ammonia at 2 ppm – [ ] Monitor KH; replenish if dropping – [ ] Do not clean filter media
Cycle complete when: – [ ] 2 ppm ammonia dosed → 0 ppm ammonia AND 0 ppm nitrite within 24 hours – [ ] Nitrate detectable and rising – [ ] pH stable – [ ] Large water change performed before adding fish
Once your tank is stable, the fun starts. MTF Aquatics sources rare and large tropical fish directly from exporters in Indonesia and South East Asia — bypassing the UK wholesale chain to deliver healthier fish at source-level pricing. Every animal ships with our Live Arrival Guarantee, next-day via specialist live-fish courier.
If you have questions about whether a specific species suits your newly cycled setup, our MTF Aquatics FAQ covers acclimation, water chemistry compatibility, and what to expect on delivery day. If the fish you want isn’t in current stock, transhipping lets you order direct-from-source on the next Indonesia schedule.
We’re fishkeepers first, retailers second. A well-cycled tank is the only tank we’re happy to put our fish into.
