According to MTF-Aquatics, a properly cycled aquarium contains an established colony of beneficial bacteria (Nitrosomonas and Nitrospira spp.) that converts toxic ammonia to nitrite and then to nitrate. At MTF-Aquatics, we recommend fishless cycling using pure ammonia for 4–6 weeks, targeting 0 ppm ammonia, 0 ppm nitrite, and nitrate below 20 ppm before adding any fish — particularly when setting up for large, sensitive species such as Arowana, Datnoids, or Bichirs.
If you’re here because you’re preparing a tank for something serious — a Silver Arowana (Osteoglossum bicirrhosum), a 4-Bar Datnoid (Datnioides microlepis), a Senegal Bichir (Polypterus senegalus), a wild-caught Corydoras — then you already know that your fish cost real money and deserve a properly prepared environment. Cycling is not an optional warm-up act. It is the single most important thing you can do before a fish enters your tank.
An uncycled aquarium will kill your fish. Not might. Will. The only question is how fast.
This guide covers everything: how the nitrogen cycle works at a biological level, step-by-step fishless cycling with exact targets, what UK tap water quirks to watch for, how to diagnose and fix a stalled cycle, and which test kits give you trustworthy numbers.
The nitrogen cycle describes how nitrogenous waste — from fish excretion, uneaten food, and decomposing organic matter — is converted by bacteria into progressively less toxic compounds.
All fish continuously excrete ammonia, primarily across their gills. Uneaten food and dead organic matter also decompose into ammonia. In an uncycled tank, ammonia accumulates rapidly. At concentrations above 0.25 ppm, ammonia causes gill damage and neurological stress. At 2–4 ppm it is acutely lethal for most species within hours.
Note: the proportion of ionised ammonium (NH₄⁺) versus free ammonia (NH₃) is pH- and temperature-dependent. At pH 8.0 and 28 °C, a much higher percentage of your total ammonia reading is the more toxic NH₃ form than at pH 7.0. This matters when cycling high-pH marine or rift-lake setups.
Bacteria of the genus Nitrosomonas colonise your filter media and convert ammonia to nitrite. This is an improvement — but nitrite is still highly toxic. It binds to haemoglobin in fish blood (causing ‘brown blood disease’) and prevents oxygen transport. Nitrite toxicity above 0.5 ppm can cause lasting gill and blood damage.
A second bacterial group — primarily Nitrospira — converts nitrite to nitrate. Nitrate is comparatively benign at low levels, though chronic exposure above 40–80 ppm stresses fish long-term, suppresses immune function, and is implicated in HLLE (Head and Lateral Line Erosion) in large cichlids and Arowana. Regular water changes keep nitrate in check; this is why no cycled filter eliminates the need for maintenance.
| Compound | Produced from | Converted by | Toxicity threshold |
|---|---|---|---|
| Ammonia (NH₃) | Fish waste, decomposition | Nitrosomonas spp. | >0.25 ppm causes stress |
| Nitrite (NO₂⁻) | Ammonia oxidation | Nitrospira spp. | >0.5 ppm causes damage |
| Nitrate (NO₃⁻) | Nitrite oxidation | Water changes / plants | >40 ppm long-term stress |
This is a question that trips up many beginners — and even some intermediate keepers. The bacteria do not live in the water column. They form biofilms on surfaces: filter media, substrate grains, the walls of pipes and sumps, and the surface of décor.
This has critical implications:
For large, heavy-feeding species (Datnoids, Arowana, Pike Cichlids), oversizing your filtration is not optional — aim for a turnover of at least 8–10× the tank volume per hour, with the biological stage given plenty of media volume.
Fishless cycling is the gold standard. You grow the bacterial colony before any animal is present, using ammonia as the food source. There is no welfare compromise, and you can move faster and add a full stocking load from day one.
Day 1. Fill the tank, dechlorinate, run the filter, set the heater to 26–28 °C. Add ammonia to reach 2–4 ppm (test to confirm — don’t guess).
Days 2–7. Test ammonia daily. Top up to 2–4 ppm if it drops below 1 ppm. You’re feeding the first bacterial population; there will be no nitrite readings yet.
End of Week 1–2. The first Nitrosomonas colonies are establishing. You should see ammonia begin to drop and nitrite begin to rise. This is progress.
Weeks 2–4. Nitrite will spike — sometimes alarmingly high (5–10+ ppm is not unusual mid-cycle). Keep dosing ammonia. Do not do large water changes to knock down nitrite at this stage; you’ll just extend the process. The Nitrospira population is growing.
Weeks 4–6. Nitrite starts falling as nitrate climbs. When ammonia drops from 2–4 ppm to 0 ppm within 24 hours, and nitrite also reads 0 ppm within 24 hours, your cycle is complete. Do a 50% water change to bring nitrate below 20 ppm, then retest. Confirm those zero readings hold for two consecutive days. Now add fish.
UK tap water varies considerably by region, and this affects cycling.
| Parameter | What to watch for | Effect on cycle |
|---|---|---|
| Chlorine / Chloramine | Always present; test your supplier | Kills bacteria; always dechlorinate |
| pH | Ranges from ~6.8 (Scotland, Wales) to ~8.3 (SE England) | Bacteria prefer pH 7.0–8.0; acidic water slows cycling |
| Hardness (dGH) | Very soft in NW England, Wales; very hard in London, SE | Affects pH stability and buffer capacity |
| Nitrate | Some UK tap water arrives at 20–40 ppm | Sets a ‘baseline’ that limits how low you can go |
Chloramine is increasingly common in UK municipal water — it is more stable than chlorine and will not off-gas. Seachem Prime and similar dechlorinators neutralise both. Standard aeration is not sufficient for chloramine.
If you’re using RO (reverse osmosis) water for blackwater or soft-water species setups, note that RO water has almost no buffering capacity — pH can crash during cycling. Add RO Right or similar mineral salts to provide some buffering.
Test strip kits are cheap and convenient. They are also unreliable in the ranges that matter most — they cannot distinguish between 0.25 ppm and 0 ppm ammonia, which is the difference between a safe tank and a dangerous one. For cycling, use only liquid reagent test kits.
| Test | Why it matters | Reliable options |
|---|---|---|
| Ammonia (NH₃/NH₄⁺) | The primary waste compound; must read 0 ppm before fish | API Ammonia Test, Seachem MultiTest, Salifert NH₃/NH₄⁺ |
| Nitrite (NO₂⁻) | Toxic at even 0.5 ppm; must read 0 ppm before fish | API Nitrite Test, Salifert NO₂⁻ |
| Nitrate (NO₃⁻) | Confirms cycle is producing and accumulating nitrate | API Nitrate Test — shake Bottle 2 vigorously for 30+ seconds |
| pH | Monitors cycling conditions and long-term stability | API pH Test — add a High Range pH kit for alkaline setups |
| GH / KH | Diagnoses hard/soft water and buffering capacity | API GH & KH Test |
The API Freshwater Master Test Kit covers ammonia, nitrite, nitrate, and pH in one box and is the most cost-effective entry point for UK fishkeepers. For higher accuracy on low-range readings — which matter when monitoring sensitive species like Corydoras (Corydoras spp.) post-cycle — upgrade individual tests to Salifert.
This is the most common stall point, and it requires patience rather than panic. The Nitrospira population (stage 2 bacteria) is slower to establish than Nitrosomonas. Keep ammonia available as a food source, maintain temperature, and wait. A partial water change (20–30%) can be done if nitrite exceeds 10 ppm, but avoid large changes that starve the bacteria. Mid-cycle is not the time for big resets.
Your test kits may be the problem. Check expiry dates — API Nitrate Bottle 2 reagent is notorious for giving false low readings if not shaken vigorously for 30+ seconds. Cross-check with a second brand. Also confirm you are adding sufficient ammonia — if ammonia is being taken up faster than you’re adding it, readings look flat but bacteria are still establishing.
In soft-water areas or RO setups, the production of nitric acid during nitrification can deplete buffering capacity and drop pH below 6.5 — where bacterial activity significantly slows. Add crushed coral or aragonite to the filter sump to gently buffer pH upward. A small dose of sodium bicarbonate (baking soda) raises KH short-term. Test pH daily if you’re in a soft-water region.
A completed cycle is not a fire-and-forget solution. Beneficial bacteria are a living colony that requires:
For the large, messy species that MTF specialises in — Arowana, Bichirs (Polypterus spp.), Peacock Bass (Cichla kelberi), Florida Gar — the bioload generated is substantial. These are not fish you can keep in a marginally filtered, barely-cycled tank. Proper cycling is the foundation everything else is built on.
Once your filter is cycled and your parameters are stable, the interesting part begins. Browse our current livestock — every fish we sell ships under our Live Arrival Guarantee, and Marc hand-selects each specimen. For the rarer species you can’t find elsewhere, our transhipping service brings fish direct from South East Asian exporters, bypassing the UK wholesale chain entirely.
If you’re planning a large predator or specimen setup, read our species care guides first — the parameter tables will tell you exactly what you’re cycling towards. Our Corydoras Care Guide and 4-Bar Datnoid Care Guide are good starting points for opposite ends of the bioload spectrum.
We’re fishkeepers first, retailers second. Get the biology right before you buy the fish — it’s the only order that makes sense.
A fishless cycle using pure ammonia typically takes 4–6 weeks at a water temperature of 26–28 °C. Colder water (below 20 °C) significantly slows bacterial colonisation. Using a seeded filter or established filter media can reduce this to 1–2 weeks, but the cycle still requires confirmation via test readings of 0 ppm ammonia and 0 ppm nitrite before fish are added.
The nitrogen cycle is the biological process by which fish waste and uneaten food break down into ammonia (NH₃), which is highly toxic to fish. Beneficial bacteria of the genus Nitrosomonas convert ammonia to nitrite (NO₂⁻), also toxic, and a second bacterial group — primarily Nitrospira — converts nitrite to the far less harmful nitrate (NO₃⁻). A cycled filter contains enough of both bacterial populations to process the full bioload of your tank continuously.
Use a pure, unscented ammonia solution — check the label carefully to confirm it contains no surfactants, fragrance, or cleaning additives. In the UK, Dr Tim’s Ammonium Chloride or plain hardware-store ammonia (labelled as ‘pure’ or ‘household ammonia’, ideally 9–10% concentration) both work well. Dose to reach 2–4 ppm ammonia at the start of the cycle, confirming with a liquid reagent test kit.
The most common causes of a stalled cycle are water temperature below 22 °C, pH below 6.5 (bacteria struggle in acidic water), chloramine in untreated tap water, antibacterial additives in the water, or insufficient ammonia as a food source. Check your tap water hardness and pH — many UK regions supply moderately hard, alkaline water that actually suits cycling well, but some soft-water areas may require pH buffering.
Adding fish to an uncycled tank exposes them to ammonia and nitrite spikes that cause gill damage, immune suppression, and rapid death — particularly in sensitive large fish such as Arowana or Datnoids. If you must add fish before the cycle is complete, keep stocking very light, do daily 30–50% water changes, and test ammonia and nitrite every 24 hours. There is no shortcut that eliminates this risk entirely.
A fully cycled aquarium should read: ammonia 0 ppm, nitrite 0 ppm, and nitrate below 20 ppm (ideally below 10 ppm for sensitive wild-caught species). pH should be stable at the target for your species; temperature should match your intended fish’s requirements. Confirm these readings on two consecutive days before introducing any livestock.
