How to Cycle a Saltwater Tank
🐠 The short version: cycling grows the bacteria that turn toxic ammonia → nitrite → nitrate, so your tank can handle the waste livestock produce. It usually takes a few weeks, and it’s finished when ammonia and nitrite both read zero. Never use a live fish to cycle — it’s the most common avoidable cruelty in the hobby. Cloudy water and a brown “uglies” phase along the way are normal.
🔧 Test against the real targets on the reef parameter chart — during cycling you’re watching ammonia and nitrite fall to zero.
What “cycling” actually means
Everything living in your tank produces waste, and that waste becomes ammonia, which is highly toxic. A healthy tank handles it with two groups of bacteria:
- One converts ammonia → nitrite (nitrite is also toxic).
- Another converts nitrite → nitrate (nitrate is far less toxic, and is removed by water changes — and used by corals).
“Cycling” is simply the weeks it takes to grow enough of those bacteria on your rock and surfaces to keep up with the waste. Until they’re established, there’s nothing to detoxify the ammonia — which is why adding livestock too early poisons it.
How to start the cycle
You need to give the bacteria something to eat — a source of ammonia:
- Bottled ammonia (dosed to a small target) is the clean, controlled method.
- Live rock brings bacteria and often a little die-off to kick things off.
- A bacterial starter culture can seed the tank and speed things along.
Set the tank up fully first — salt mixed to about 35 ppt (SG ≈ 1.026), heater running, flow on — then introduce the ammonia source and wait. Keep the lights low; you’re growing bacteria, not coral.
What to watch, and for how long
Test every few days and you’ll see the classic pattern:
- Ammonia rises, then falls as the first bacteria take hold.
- Nitrite rises, then falls as the second group catches up.
- Nitrate appears and climbs — the sign the cycle is completing.
The whole process usually takes a few weeks, occasionally up to a couple of months. It’s genuinely done when your tank can take a dose of ammonia to zero ammonia and zero nitrite within about a day, leaving only nitrate. Those two zeros are the finish line — see the parameter chart, where ammonia and nitrite are the only readings that should be zero.
The “uglies” are normal
During and just after cycling, most tanks go cloudy for a while (a harmless bacterial bloom) and grow a brown diatom film — the “uglies.” This is a rite of passage, not a failure. It passes as the tank matures. Don’t scrub obsessively or dose chemicals to fight it; let a maturing tank and (later) a cleanup crew handle it. More on that in beating reef tank algae and pests.
When it’s done
Cycled means: ammonia 0, nitrite 0, nitrate present. Do a water change to bring nitrate down to a sensible level, confirm salinity and temperature are stable, and then add your first saltwater fish — slowly, one at a time. Corals wait until the tank is more mature still.
Sources & further reading
- Coralkin — Reef parameter target chart — the ammonia/nitrite/nitrate targets, sourced.
- Coralkin — Cycling questions — quick answers to the common cycling worries.
- Bulk Reef Supply — the nitrogen cycle and starting a saltwater tank.
- WetWebMedia — marine cycling and establishing biological filtration.
Common questions
How long does it take to cycle a saltwater tank?
Usually a few weeks, sometimes up to a couple of months. It is done when your tank can convert a dose of ammonia to zero ammonia and zero nitrite within about 24 hours, leaving only nitrate. Do not rush it — the timeline depends on temperature, the bacteria you seed with, and luck.
How do I know when my tank is cycled?
When ammonia reads zero and nitrite reads zero, with nitrate present. The classic test: add a small ammonia source, and if within a day both ammonia and nitrite are back to zero, the bacteria are established and the tank is cycled.
Can I add fish to speed up the cycle?
No — and please do not. A fish is not a cycling tool; cycling with a live fish means poisoning it with ammonia and nitrite. Cycle with a bottled ammonia source or live rock instead, then add fish once it is finished.
My tank went cloudy or brown during cycling — is that bad?
No, that is normal. A bacterial bloom (cloudy water) and a brown diatom film (the "uglies") are common during and just after cycling. They pass on their own as the tank matures — resist the urge to scrub or dose your way out of them.
Do I need to do a water change during cycling?
Usually not during the cycle itself — you want the bacteria to have ammonia to feed on. Once cycled, a water change to bring nitrate down to a sensible level is a good idea before adding livestock.