How to cycle a new aquarium (nitrogen cycle)

Cycling means growing a colony of nitrifying bacteria in a new aquarium's filter so it can turn toxic ammonia into nitrite and then into far safer nitrate. Without fish, dose ammonia to 2–3 mg/L and test every 2–3 days. Most unseeded tanks are ready in 2–8 weeks, once a full dose disappears within 24 hours and nitrite reads zero.

What is the nitrogen cycle in an aquarium?

It is the chain that turns fish waste into something fish can live with. Fish excrete ammonia, and leftover food, rotting leaves and some nutrient soils add more. A first group of microbes, the ammonia oxidisers such as Nitrosomonas and Nitrosospira (plus some archaea, especially at low pH), converts ammonia into nitrite. A second group converts nitrite into nitrate, which builds up slowly and leaves the tank through water changes and plant uptake.

A new tank has almost none of these microbes, so in the first weeks ammonia and then nitrite climb. Fish added at that point are poisoned, which hobbyists call new tank syndrome. Cycling simply means building the colony first, mostly on the filter media and other surfaces, before the tank carries a real fish load.

One correction to older books: the nitrite step is often credited to Nitrobacter, but a 1998 study of freshwater aquaria led by Timothy Hovanec found Nitrospira-like bacteria doing the work and no detectable Nitrobacter. Aquaculture references still list both. The method below does not change either way.

Why is ammonia so dangerous to fish?

Because part of it is un-ionised ammonia (NH3), roughly 100 times more toxic than the ammonium ion (NH4+) that makes up the rest. Test kits read both together as total ammonia, and the toxic share rises steeply with pH and temperature.

Fish tissue starts to suffer at around 0.05 mg/L NH3, and nitrite can do harm from about 0.1 mg/L. The same test reading is therefore far worse in hard, alkaline water than in soft, acidic water, and neither is safe in a tank that is still cycling. Nitrate, the end product, is much less harmful and is kept down with water changes.

How long does it take to cycle a tank?

Usually 2–8 weeks for a tank started from scratch, but reputable sources give very different numbers because seeding, pH and temperature change the pace so much:

Plan for about a month and treat anything faster as a bonus. The usual advice is that warmer water above pH 7 cycles faster, yet some nitrifiers (Nitrotoga, for example, does best near pH 6.8 and 22 °C) prefer cooler, softer water. There is no single ideal; the firm rule is not to let pH slide below about 7 during a fishless cycle.

• Dr Tim's Aquatics: about 30–45 days fishless with no additives, under a week with its own live-bacteria product.
• UF/IFAS Extension: a new biofilter needs about 6–8 weeks.
• ADA: waits 5–8 weeks before adding livestock to a new Nature Aquarium.
• Aquarium Science: anywhere from 2 to 20 weeks for planted tanks.
• Wikipedia: from instant (with a well-seeded filter) to 6 months.

How do you do a fishless cycle?

Feed the empty tank pure ammonia in place of fish and let the bacteria grow on it. It is the humane way to cycle and much easier to control than cycling with fish.

On the dose, sources disagree: one Aquarium Science page argues for much higher ammonia levels. We use 2–3 mg/L, the conservative figure from Dr Tim's (a company run by a nitrifier researcher), because very high ammonia or nitrite holds the bacteria back rather than speeding them up.

1. Fill the tank with dechlorinated water and run the filter and heater non-stop from day one. The filter media will hold much of the colony.
2. Add pure ammonia (ammonium chloride, or household ammonia with no surfactants or scent) to about 2–3 mg/L. Do not go past about 5 mg/L.
3. Test ammonia and nitrite every 2–3 days. When ammonia falls, top it up with roughly half the first dose.
4. Expect ammonia to start dropping and nitrite to appear after about 9–12 days; nitrite usually peaks around days 14–20. Keep dosing through this slow stretch.
5. If nitrite rises above about 5 mg/L or pH falls below about 7, do a partial water change; both stall the bacteria.
6. Once the tank passes the test in the next section, do a large water change to bring nitrate down, then add livestock gradually.

How do you know a tank is cycled?

The tank is cycled when it clears a full 2–3 mg/L ammonia dose within 24 hours, with no nitrite left and nitrate still climbing. Check all three readings:

Remember that a tank is cycled for the load it was fed. A colony grown on a small dose can still be overwhelmed by a big group of fish added at once, so stock in stages and feed lightly at first. In planted tanks, test at the same time each day: ammonia can read high in the morning and be gone by evening.

• Ammonia: a full dose reads zero 24 hours later.
• Nitrite: zero.
• Nitrate: rising, which shows the second step is working.

Why has my tank stopped cycling?

A stalled cycle almost always comes down to too much ammonia, too much nitrite or a pH that has dropped. Fix the cause with water changes rather than adding more of anything.

• Overdosing: above about 5 mg/L, ammonia or nitrite inhibits the bacteria. Water-change down and go back to 2–3 mg/L doses.
• Falling pH: nitrification produces acid, and below about pH 7 it slows and can stop. Partial water changes restore KH and pH.
• Lost media: the filter holds much of the colony, so never replace it wholesale; rinse it gently in old tank water when it clogs.

Can plants replace cycling in a planted tank?

No. Plants help, but they do not replace cycling. Fast growers such as stem plants, Egeria and Limnophila take up ammonia and nitrate and steady a new tank, but fresh plants often melt back and need weeks to root, while nutrient soil and dying leaves keep releasing ammonia. That makes a planted start-up less predictable than a bare fishless cycle.

• Nutrient aquasoil leaks ammonia for the first weeks: change 25–50 % of the water several times a week for the first 3–4 weeks.
• Keep the light to about 6 hours a day while cycling to hold algae back. CO₂ is not needed for the bacteria, although Tropica runs it from day one in planted tanks.
• Wait until ammonia holds at or below about 0.25 mg/L and plants push new leaves, then do a big water change before adding fish.
• Algae-grazing shrimp and snails can go in early (Tropica says as soon as possible) once ammonia and nitrite read zero; fish come later.

Do bottled bacteria and seeded filter media speed up cycling?

Yes, when the source is good. Mature filter media from an established, healthy tank already carries a working colony and can shorten cycling dramatically. Bottled live nitrifiers are less consistent: independent tests summarised on Wikipedia rated FritzZyme TurboStart and Tetra SafeStart well and several other products poorly. Whatever you add, trust the test rather than the label: a full ammonia dose to zero in 24 hours, with zero nitrite.

What should you do after the tank is cycled?

Lower the nitrate, then stock slowly so the colony can grow with the load.

• Do a large water change before the first fish go in.
• Add fish in stages and feed lightly for the first weeks.
• Change about 25–33 % of the water weekly and test nitrate.
• Once a month, rinse filter media gently in old tank water instead of replacing it.

이 가이드의 식물과 생물

• 로탈라 / 레드 스템 군락 (Rotala rotundifolia / Ludwigia palustris)
• 체리새우 (네오카리디나 생이새우과) (Neocaridina davidi)
• 범블비/너라이트 스네일 / 램즈혼 달팽이 (Vittina natalensis (syn. Neritina natalensis) / Planorbarius corneus)

관련 가이드

• Balancing light, CO₂ and fertiliser in a planted tank
• Aquarium algae: identify the type and fix the cause
• Nature Aquarium 스타일: 레이아웃 규칙과 세팅 방법

출처

1. FA16/FA031: Ammonia in Aquatic Systems — UF/IFAS Extension (University of Florida)
2. Nitrospira-Like Bacteria Associated with Nitrite Oxidation in Freshwater Aquaria (Applied and Environmental Microbiology 64(1):258–264) — American Society for Microbiology
3. Fishless Cycling — DrTim's Aquatics
4. Fishless cycling — Wikipedia
5. 2.15. Cycling a Planted Aquarium — Aquarium Science
6. 17.2.1. Blackwater Biotope — Aquarium Science
7. Starting a new aquarium — Tropica Aquarium Plants
8. Nature Aquarium Starting from Zero — ADA (Aqua Design Amano)
9. How to set up an underwater jungle aquarium — Practical Fishkeeping

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