To cycle a saltwater tank, you add a source of ammonia and give beneficial bacteria time to grow until the tank turns that ammonia into nitrate on its own. Most reef tanks take somewhere between 2 and 8 weeks, and you know the cycle is done when a dose of ammonia drops to near zero within about 24 hours and nitrite reads near zero too.
What the nitrogen cycle is
Fish, food, and decaying matter all release ammonia. In a new tank there is nothing to remove it, so it builds up. The cycle is the process of growing two groups of bacteria that clean it up for you:
- Ammonia to nitrite. The first group of bacteria uses ammonia and gives off nitrite.
- Nitrite to nitrate. A second group uses that nitrite and gives off nitrate.
Nitrate is much less harmful than ammonia and nitrite, and it stays in the water until water changes, a skimmer, a refugium, or algae and corals remove it. A cycled tank is one where enough of these bacteria live on the rock, sand, and filter surfaces to keep up with the ammonia you produce. Because they live on surfaces, a water change does not "reset" a cycle.
Before you start
Fill the tank with mixed salt water and run your return pump, powerheads, and heater as if livestock were already in it. Bacteria work best in warm, oxygen-rich, steady water, so aim for:
- Temperature: inside the 74 to 82 °F ideal range. Many reefers cycle at the warmer end, around 78 °F.
- Salinity: 1.025 to 1.027 SG (about 35 ppt).
- pH: 7.9 to 8.4. Nitrifying bacteria slow down in low pH water.
- Flow: good surface movement so the water stays well oxygenated.
Some reefers keep lights off or on a short schedule during the cycle to limit early algae, others run their normal schedule. Both work.
Ammonia sources
The bacteria need food. You have three common ways to provide it.
Pure ammonia
Dosing ammonium chloride or plain household ammonia (no fragrance, no surfactants) is the most controlled method. A common approach is to raise ammonia to about 1 to 2 ppm, then re-dose back to that level each time it drops. You know exactly how much went in, and you add no phosphate.
Ghost feeding
Ghost feeding means adding fish food to an empty tank and letting it break down. It is simple, but harder to control, and decaying food also adds phosphate, which can feed algae later. If you choose it, add a small pinch every day or two.
Live rock
Live rock already carries bacteria, and some die-off as it settles releases ammonia on its own. Cured live rock can shorten the cycle a lot. The trade-off is hitchhikers you may not want, such as aiptasia or certain crabs. See common reef tank pests for what to look out for.
Dry rock is clean and pest free, but it starts with no life on it and usually takes longer to cycle.
Bottled bacteria and other accelerators
Bottled nitrifying bacteria can speed up a cycle, especially on dry rock. Results vary, and how much they help is still debated in the hobby. If you use one, dose ammonia alongside it so the bacteria have food, and keep testing rather than assuming the cycle is done on a set day.
A cup of sand, a piece of rock, or filter media from an established, healthy tank also seeds a new one well. It can bring pests too, so use a source you trust.
What to test and how often
You only need three tests during the cycle: ammonia, nitrite, and nitrate. Keep an eye on pH, temperature, and salinity as well, since those affect how fast the bacteria grow.
| Stage | What you usually see | How often to test |
|---|---|---|
| Week 1 | Ammonia high, nitrite starting to rise | Every 2 to 3 days |
| Middle | Ammonia falling, nitrite high | Every 2 to 3 days |
| Late | Ammonia and nitrite falling, nitrate rising | Every 1 to 2 days |
| Final check | Dose ammonia, test 24 hours later | Daily for a few days |
Write every result down with the date. The pattern tells you more than any single reading: an ammonia peak, then a nitrite peak, then nitrate climbing as both fall.
Tip: Test at about the same time of day each time, and from the same spot in the tank, so your readings are easy to compare.
How to know the cycle is done
A common test for a finished cycle looks like this:
- Dose ammonia to about 1 to 2 ppm.
- Test again roughly 24 hours later.
- If ammonia reads near zero (inside the 0 to 0.1 ppm ideal range) and nitrite reads near zero too, the tank is processing ammonia on its own.
Repeat the check over two or three days to be sure it holds. Nitrate will usually be building up by now, which is expected. Many reefers do a partial water change once the cycle is complete to bring nitrate down before adding livestock, then keep it in the 5 to 20 ppm ideal range over time. For ways to manage it, see how to lower nitrate and phosphate.
A note on nitrite in salt water
Nitrite is a big worry in freshwater tanks, but far less of a concern in salt water, because the chloride in sea water reduces how much nitrite fish take up. That is why Reefability's default ideal range for nitrite is 0 to 1.5 ppm. Falling ammonia is the more important signal, though near zero nitrite at the end shows the second group of bacteria is established.
How long does it take to cycle a reef tank?
There is no fixed answer, and every tank is different. As rough ranges:
- Dry rock with pure ammonia only: often 4 to 8 weeks.
- Dry rock with bottled bacteria: often 1 to 4 weeks.
- Cured live rock or seeded material: sometimes 1 to 2 weeks, occasionally faster.
Cooler water, low pH, and lots of dry rock tend to slow it down. If ammonia or nitrite sits still for a week, check pH and temperature, then give it more time before changing anything else.
The ugly phase
Once the cycle finishes, most new reef tanks go through an "ugly phase." It is a normal part of a tank maturing: nutrients are uneven, and different algae and bacteria take turns before things settle.
What you might see, often in roughly this order:
- Diatoms: a brown, dusty film on sand and glass. It usually fades on its own within a few weeks.
- Cyanobacteria: red, maroon, or dark green slime sheets, often in low flow spots. Better flow and steady nutrient control usually bring it down.
- Hair algae: green strands on the rock. Manual removal and a cleanup crew help, along with keeping nitrate and phosphate in check.
A calm approach works best. Siphon out what you can during water changes, keep flow strong, and avoid large swings or strong chemical treatments early on. The phase can last a few weeks to a few months, and patience is usually the best tool.
Adding livestock slowly
Bacteria grow to match the amount of ammonia available, so add livestock in small steps to let them keep up.
- Cleanup crew first. A few snails and small hermit crabs help with diatoms and leftover algae. Start with a small number and add more if there is still food for them.
- Hardy fish next. One or two hardy fish that are known to do well in new tanks. Many reefers quarantine new fish first.
- Wait between additions. Give the tank about 2 to 4 weeks between new fish, and test ammonia a day or two after each one.
- Corals once things are steady. Hardy soft corals are a common first choice once parameters have been stable for a while. Before adding anything new, check it against your current stock and your ideal water parameters.
A small ammonia bump after an addition that settles back to zero is just the bacteria catching up. Feed lightly and keep testing.
Tracking your cycle in Reefability
Reefability lets you log ammonia, nitrite, and nitrate results as you go and see them on a chart against your ideal ranges, which makes the ammonia and nitrite peaks easy to spot. You can ask Reefer AI questions about your own readings, set test reminders as tasks, and use the livestock compatibility checker when you are ready to stock.
Frequently asked questions
Can I add fish to speed up the cycle?
It is possible, but it puts the fish through high ammonia while the bacteria grow. Most reefers today prefer a fishless cycle and add fish once the tank processes ammonia on its own.
Do I need to do water changes during the cycle?
Usually not, unless ammonia goes well above 2 to 3 ppm and seems stuck. Very high ammonia can slow the bacteria. A partial water change at the end of the cycle is common to lower nitrate before livestock goes in.
Is the ugly phase a sign something went wrong?
No. It is a normal stage in a new tank. Diatoms, cyano, and some hair algae are all common in the first months. Steady water changes, good flow, and a modest cleanup crew help it pass.