What "cycling" actually means
Cycling grows the beneficial bacteria that make your tank safe for life. Those bacteria convert toxic waste through the nitrogen cycle: ammonia → nitrite → nitrate. Ammonia and nitrite are lethal to fish and coral even in tiny amounts; nitrate is far less toxic and gets removed by water changes. Until that bacterial colony is established, the tank can't process waste and anything you add will suffer.
How long does it take?
Usually 2 to 6 weeks. Bottled bacteria and live rock can shorten it; a bare, sterile start takes longer. You can't reliably rush biology — the cycle is done when the tests say so, not when the calendar does.
Step by step
- Set up and mix saltwater. Get salinity to ~1.025, temperature to 76–78 °F, and run your pumps, heater, and skimmer.
- Add a bacteria home. Live rock or dry rock plus a bottled nitrifying bacteria product gives the colony somewhere to grow.
- Add an ammonia source. Dose bottled ammonia to ~2 ppm (cleanest method), or use a small piece of raw shrimp. The bacteria need food to multiply.
- Test every few days. Watch ammonia rise and fall, then nitrite rise and fall, then nitrate appear. That sequence is the cycle.
- Wait for the finish line. The cycle is complete when ammonia and nitrite both read 0 within 24 hours of dosing ammonia, and nitrate is present.
- Water change, then stock slowly. Do a water change to bring nitrate down, then add a couple of hardy fish — not a full tank at once.
Common mistakes
- Adding livestock before ammonia and nitrite hit zero — the classic "new tank" livestock loss.
- Cleaning too aggressively mid-cycle and knocking back the bacteria you're trying to grow.
- Trusting the calendar over the test kit.
Track the cycle so you know exactly when it's done
The cycle is a sequence of readings, and it's easy to lose track of where you are. Log your ammonia, nitrite, and nitrate in TankMate and watch the curve turn over — you'll see the moment it's safe to stock, with the whole history in one place. It's free on Google Play.