According to MTF-Aquatics, fishless cycling a new aquarium takes 4–8 weeks using pure ammonia dosed to 2–4 ppm, monitored daily with a liquid reagent test kit. The cycle is complete when ammonia and nitrite both drop to 0 ppm within 24 hours of a 2 ppm ammonia dose, confirming the biological filter can handle a full stock load.
If you are setting up a tank for a Bichir (Polypterus spp.), a large Pleco, an Arowana, or any of the predatory centrepieces that MTF specialises in, the nitrogen cycle is not a beginner formality — it is the difference between a thriving specimen and a dead one inside a week. Large, messy fish produce ammonia at a rate that would overwhelm an uncycled filter before you notice anything is wrong. A 90 cm Polypterus endlicheri eating a weekly meal of Hikari Massivore generates more ammonia per 24 hours than a community tank of 20 tetras.
This guide is written for the hobbyist who is preparing a serious setup: 4 ft tanks and above, large-format external filters, and fish that represent a significant investment. We will not gloss over the timeline or soften the numbers.
The nitrogen cycle is the biological process by which toxic ammonia (NH₃) — produced by fish waste, uneaten food, and decomposing matter — is converted into progressively less toxic compounds by colonies of nitrifying bacteria living in your filter media and substrate.
The two-stage conversion works like this:
| Stage | Bacteria | Converts | To |
|---|---|---|---|
| Stage 1 | Nitrosomonas spp. | Ammonia (NH₃) → | Nitrite (NO₂⁻) |
| Stage 2 | Nitrospira spp. | Nitrite (NO₂⁻) → | Nitrate (NO₃⁻) |
Ammonia is acutely toxic to fish at concentrations above 0.25 ppm (as NH₃ at pH 7.5 and 25 °C). Nitrite is also directly toxic — it binds haemoglobin, causing ‘brown blood disease’. Nitrate is relatively harmless at levels below 40 ppm for most species and is managed through regular water changes.
The goal of cycling is to grow enough of both bacterial colonies to process the full ammonia load of your stocked tank within 24 hours.
Before dosing a single drop of ammonia, the following must be in place:
At MTF-Aquatics, we recommend the ammonia-dosing method over fish-in cycling or the older raw prawn method. It is cleaner, more controllable, and — critically — gives you precise data to act on.
Fill the tank fully, run the filter and heater, and add dechlorinator at the manufacturer’s dose. Wait 30 minutes before adding ammonia.
The target starting concentration is 4 ppm ammonia (NH₃/NH₄⁺ total).
As a practical rule for 10% pure ammonia solution:
| Tank Volume | Approximate Dose for 4 ppm |
|---|---|
| 100 litres | ~0.4 ml |
| 200 litres | ~0.8 ml |
| 300 litres | ~1.2 ml |
| 500 litres | ~2.0 ml |
| 1,000 litres | ~4.0 ml |
These are starting estimates — always verify with your test kit immediately after dosing and adjust accordingly. Ammonia concentration varies between brands.
Test ammonia, nitrite, and pH every 24 hours, at the same time each day. Keep a written log — this is not optional. When you see ammonia start to fall (usually days 7–14), top it back up to 2 ppm to keep the bacteria fed. Do not let ammonia crash to 0 ppm; the bacteria will starve.
Timelines vary by temperature, tank volume, filter media, and whether you are seeding from an established system. This is what a typical 200-litre tank at 26–28 °C looks like:
| Day Range | Ammonia (ppm) | Nitrite (ppm) | Nitrate (ppm) | What’s Happening |
|---|---|---|---|---|
| Day 1 | 4.0 | 0 | 0 | Ammonia dosed; bacteria not yet present |
| Days 2–7 | 3.0–4.0 | 0–0.25 | 0 | Ammonia stable; Nitrosomonas beginning to colonise |
| Days 7–14 | 2.0–3.5 | 0.25–2.0 | Trace | Nitrite rising — Stage 1 bacteria establishing |
| Days 14–21 | 1.0–2.5 | 2.0–5.0+ | 5–10 | Nitrite spike peak — Nitrospira lagging behind |
| Days 21–35 | 0.5–1.5 | Falling | 20–40 | Nitrite dropping; nitrate climbing |
| Days 35–42 | 0 within 24 hrs | 0 within 24 hrs | 40–80 | Cycle complete — do a large water change before adding fish |
A note on the nitrite spike: API’s nitrite test kit maxes out at 5 ppm. During the peak spike, you may see bright purple that’s off the chart. Dilute your tank water 1:1 with distilled (not tap) water, run the test, and double the reading. Readings of 10–20 ppm at peak are normal and not a cause for alarm — there are no fish present.
The cycle is complete when both of the following are true:
Do this test twice on consecutive days to confirm. Nitrate should be detectable (typically 20–80 ppm at this point) — this confirms the full conversion chain is working.
Before adding fish, perform a 50–80% water change to bring nitrate below 20 ppm. Large predators and sensitive wild-caught species are particularly intolerant of elevated nitrate at introduction.
A stall is not the same as a slow cycle. If your readings have not changed in 7+ days, work through this checklist:
Your bacteria starved. Top ammonia back to 2 ppm immediately and test again in 24 hours. The colony may have reduced in size but should recover within 3–5 days.
Nitrification is severely impaired below pH 6.5 and slows noticeably below 7.0. As nitrification proceeds, it acidifies the water naturally — this is the most common hidden stall cause. Perform a 30% water change with dechlorinated tap water, check your tap’s pH (most UK tap water runs pH 7.2–7.8), and retest. Do not chase pH with additives during cycling.
Nitrosomonas and Nitrospira reproduction roughly halves with every 10 °C drop below optimal. Check your heater is actually holding temperature with a calibrated thermometer — not just the dial setting. A 300 W heater may struggle to hold 27 °C in a cold room during a UK winter.
Several UK water authorities use chloramine rather than chlorine. Chloramine does not off-gas and requires a dechlorinator that specifically breaks the chloramine bond. If you have been doing top-up water changes with a basic dechlorinator, you may be continuously poisoning your bacterial colony. Switch to Seachem Prime (5 ml per 200 litres) and re-dose with every water change.
If you have rinsed your filter media under a tap at any point during the cycle, you have likely killed a significant portion of the bacterial colony with chlorine. Always rinse biological media in a bucket of tank water only.
If you or anyone you know has an established, healthy tank, ask for a handful of their used biological media — a sponge, a cup of ceramic rings, or even a thin layer of substrate. Placed directly into your new filter, this can cut cycle time to 10–14 days. This is the method Marc uses when setting up new holding systems at MTF: seed heavily and monitor carefully.
Set your heater to 28–30 °C during the fishless cycle. Since there are no fish, you are not risking heat stress. Drop back to your species’ target temperature before introduction.
Commercially bottled nitrifying bacteria (Tetra SafeStart Plus, Dr Tim’s One & Only) are often combined with ammonia dosing. Results are genuinely variable — some hobbyists report a 1–2 week reduction; others see no effect. They are not a substitute for the process, but they are not snake oil either. If you use them, add the full recommended dose immediately and do not do a water change for 7 days after addition.
If you are cycling a tank destined for a large predatory or wild-caught specimen — an Arowana, a Bichir, a Wolf Fish, or a stingray — there are a few additional considerations beyond the standard guide:
Target parameters before introduction:
| Parameter | Target Range | Notes |
|---|---|---|
| Ammonia | 0 ppm | Non-negotiable |
| Nitrite | 0 ppm | Non-negotiable |
| Nitrate | <20 ppm | Wild-caught fish are particularly sensitive at introduction |
| pH | Species-specific | Check care guide for your species |
| Temperature | Species-specific | Match exactly before adding fish |
| dGH | Species-specific | Critical for stingrays; RO blending may be required |
For more on species-specific parameter requirements, see our Bichir Care Guide (Polypterus) and Royal Pleco (L190 / L191) Care Guide.
Stock lightly on introduction. Even a fully cycled filter can be overloaded if you add the full stocking in one go. A 6 ft predator tank with a target load of one large Arowana plus two Datnoid tankmates should be introduced over 2–3 weeks, testing daily for the first week after each addition.
Wild-caught fish have zero tolerance. Fish sourced direct from Indonesian and South East Asian exporters — as MTF’s livestock is — will have spent time in transit. Their immune systems are stressed. Ammonia and nitrite readings must be zero; do not round down a 0.25 ppm ammonia reading and assume it is ‘fine for now’.
Every new fish added to a display tank should pass through quarantine first — a minimum of 4 weeks in a separate, cycled system. This is non-negotiable if you are keeping expensive specimens. A quarantine tank can be cycled using the same fishless method described above, then maintained by dosing a small amount of ammonia (enough to keep the filter fed) between fish arrivals.
For guidance on what to look for during quarantine, see our MTF Care Guides collection for species-specific health indicators.
When the cycle is complete and parameters are stable, your tank is ready for the livestock it was built for. Browse our current stock of rare and large tropical fish — every animal leaves us health-checked and ships with our Live Arrival Guarantee. If you have questions about whether your setup is ready for a specific species, reach out directly — we’d rather answer a question before the order than deal with an avoidable problem after it.
Browse MTF Current Stock & Live Auctions →
At MTF-Aquatics, we recommend budgeting 4–8 weeks for a fishless ammonia cycle at typical UK room temperature (18–22 °C). Warmer water (26–28 °C) can accelerate this to 3–4 weeks. Tanks cycling at temperatures below 18 °C may take 10+ weeks because nitrifying bacteria (Nitrosomonas and Nitrospira) reproduce significantly more slowly in cold water.
At MTF-Aquatics, we recommend dosing pure, unscented household ammonia (9–10% concentration) to achieve a reading of 2–4 ppm in your tank. Use the calculator: for a 200-litre tank at 10% ammonia, approximately 0.5 ml raises ammonia by roughly 1 ppm. Dose to 4 ppm on day 1, then top up to 2 ppm each day thereafter once the cycle is under way.
Use pure ammonia solution with no surfactants, perfumes, or dyes — shake the bottle; if it foams, it contains surfactants and is not suitable. In the UK, unscented ‘Janitor’s Ammonia’ or laboratory-grade ammonium chloride (NH₄Cl) powder are both reliable. Avoid any product labelled ‘sudsy’ or ‘lemon-scented’. Seachem Ammonia Alert cards can help confirm your baseline before dosing.
According to MTF-Aquatics, the progression runs: Week 1–2, ammonia rises to 4 ppm and nitrite remains at 0; Week 2–3, nitrite spikes sharply (often above the API kit’s 5 ppm scale — dilute your sample 1:1 with distilled water and double the reading); Week 3–6, nitrite falls and nitrate climbs; Cycle complete when both ammonia and nitrite read 0 ppm within 24 hours of a 2 ppm ammonia dose.
A stalled cycle is almost always caused by one of four things: ammonia depleted to 0 ppm (bacteria starve), pH below 7.0 (nitrification slows sharply below pH 6.5), water temperature below 18 °C, or chloramine in tap water that wasn’t neutralised with a dechlorinator that specifically breaks the chloramine bond. Check all four parameters before adding more ammonia, and use a dechlorinator rated for chloramine (e.g. Seachem Prime).
According to MTF-Aquatics, the two most reliable accelerators are: (1) seeding your new filter with mature media — sponge, bio-balls, or ceramic rings from an established tank — which can cut cycle time to 1–2 weeks; and (2) raising water temperature to 28–30 °C during the fishless cycle (safe without fish present). Adding a commercially bottled bacterial culture (Tetra SafeStart Plus or Dr Tim’s One & Only) alongside ammonia dosing can also reduce cycle time by 1–2 weeks, though results are variable.
