How to Cycle a Reef Tank: The Nitrogen Cycle Explained
Cycling a reef tank means establishing the bacterial colonies that convert toxic ammonia into harmless nitrate. Get this wrong and everything you add to the tank will die. Here is the full process done right.
The nitrogen cycle is not a reef tank concept — it is a fundamental principle of all aquatic life. What makes it critical for reef keepers is that the two intermediate compounds in the cycle, ammonia and nitrite, are lethal to coral and invertebrates at concentrations that fish can sometimes survive. A tank that has not cycled is a tank that will kill everything you put in it.
This guide explains what the nitrogen cycle is, how to run it correctly in a new tank, what to test and when, and how to know when the cycle is genuinely complete.
What the nitrogen cycle actually is
Any organic material in water — fish waste, uneaten food, decaying tissue — breaks down and releases ammonia (NH₃). Ammonia is highly toxic. In a mature tank, two groups of bacteria process it:
- Nitrosomonas (and relatives): Convert ammonia (NH₃/NH₄⁺) into nitrite (NO₂⁻). Nitrite is also highly toxic — sometimes more toxic to invertebrates than ammonia itself.
- Nitrospira (and relatives): Convert nitrite (NO₂⁻) into nitrate (NO₃⁻). Nitrate is far less toxic and accumulates until you export it through water changes, refugium macroalgae, or denitrifying bacteria in deep sandbeds.
The cycle is complete when both bacterial colonies are established and large enough to process waste as fast as it is produced. Until that point, ammonia or nitrite — or both — will be detectable in the water.
Why this matters more in a reef tank than a fish-only tank
Many fish can tolerate low levels of ammonia and nitrite for short periods. Coral cannot. Even brief ammonia spikes above 0.5 ppm can bleach coral polyps. Ornamental shrimp, snails, and starfish are similarly sensitive. Running an uncycled tank is not a "slow start" — it is a guarantee of early livestock death and wasted money.
Methods for cycling a new reef tank
Fishless ammonia dosing (recommended)
This is the most reliable and humane method. You add pure ammonia to the tank (no surfactants — the bottle should foam minimally when shaken) to feed the developing bacterial colonies without adding any livestock that would suffer through the process.
Dr. Tim's Ammonium Chloride is the standard product used for this. It comes with dosing instructions calibrated to bring your tank to 2 ppm ammonia.
Bottled bacteria (fastest)
Products like Dr. Tim's One-and-Only, Fritz Turbo Start 900, and Brightwell Aquatics MicrōBacter7 contain live nitrifying bacteria in suspension. When added with an ammonia source, they can compress the cycle from 4–6 weeks down to 1–2 weeks. Quality varies between products and batches, and they should be treated as an accelerant rather than a replacement for the full cycle process.
Live rock seeding
Adding a piece of cured live rock from an established reef tank imports bacteria directly. This is effective but carries the risk of introducing pests — aiptasia anemones, flatworms, aptly named "bad hitchhikers" — along with the beneficial bacteria. If you go this route, inspect the rock carefully and dip it before adding it to your display tank.
Step-by-step fishless cycle with ammonia
The following assumes you have already set up your tank with rock, sand, salt water mixed to 1.025 sg, heater set to 78°F, and circulation running. Do not add livestock at any point during this process.
- Day 1: Dose ammonia to 2 ppm. Confirm with a test kit (API, Salifert, or Hanna checker). Note the reading and the date.
- Days 2–7: Test ammonia and nitrite every 2–3 days. Initially you will see ammonia holding steady or rising. No nitrite yet is normal in the first week.
- Week 2: Nitrite should begin to appear. This means Nitrosomonas colonies are establishing. Ammonia may start to drop. Continue dosing small amounts of ammonia to keep the bacterial food source available.
- Weeks 2–4: Nitrite will rise, sometimes to very high levels (5–20 ppm is not unusual). This is normal. Do not panic and do not do a water change — it will only extend the cycle.
- Week 4+: Nitrite begins to fall as Nitrospira colonies establish and start processing it. Nitrate accumulates. Ammonia is being processed quickly.
- Final test: Dose 2 ppm ammonia. Check again in 24 hours. If ammonia reads 0 and nitrite reads 0, the tank is cycled.
What "cycled" actually means
A cycled tank processes 2 ppm of ammonia to zero detectable ammonia and zero detectable nitrite within 24 hours. Nitrate will be present — that is expected. Do a 30–50% water change to knock nitrate down before adding your first livestock, then add slowly.
Do not rely on a single "good" reading. Test the same dose on two consecutive days. Bacterial colonies can be present but still small enough that a larger ammonia load would overwhelm them.
Typical timeline
- Fishless ammonia only: 4–6 weeks from scratch.
- Fishless ammonia + bottled bacteria: 1–3 weeks, depending on product quality and dosage.
- Seeded with live rock from established tank: 1–2 weeks, assuming significant seed material.
Temperature affects bacterial growth rate significantly. Running the tank warm (78–80°F) during the cycle speeds colonization. Cold water (below 74°F) will noticeably slow the process.
Mini-cycles after adding animals
Even after a complete cycle, adding new livestock — especially fish — will temporarily spike ammonia as the bacterial population adjusts to the increased bioload. This is called a mini-cycle, and it is normal. The key is to add livestock slowly (one or two items at a time, spaced two to four weeks apart) so that mini-cycles stay small and self-resolving. Adding five fish at once to a newly cycled tank will cause a full ammonia crash.
Monitor ammonia and nitrite for the first two weeks after any significant addition. If levels spike above 0.25 ppm, do a partial water change and hold off on adding anything else.
What to test during the cycle
- Ammonia: Salifert or Hanna HI700 checker. Test every 2–3 days during the cycle, daily during the final verification stage.
- Nitrite: API or Salifert. Same frequency as ammonia — these two travel together and tell the full story of where the cycle is.
- Nitrate: Weekly is sufficient. Its appearance confirms the second stage of the cycle is running.
- Salinity and temperature: Check weekly — evaporation can concentrate salt if you do not have an ATO running.
