Marine Tank Cycling: A Step-by-Step Guide

Every fish, every uneaten pellet and every dying coral produces ammonia. Ammonia is lethal even at 0.25 ppm. Bacteria called Nitrosomonas convert ammonia into nitrite (also toxic). A second group, Nitrobacter / Nitrospira, convert nitrite into nitrate. Nitrate is tolerated up to roughly 20 ppm in a reef and 40 ppm in a fish-only system.

A 'cycled' tank has enough of both bacteria colonies to process the ammonia your livestock produces within 24 hours.

Live rock from an established tank is the fastest route — it already carries the bacteria. Expect a cycle of 1–2 weeks and minor algae blooms as it recovers from transport.

Dry rock is cleaner, pest-free and cheaper, but you're growing the colony from scratch. Expect 4–6 weeks. You'll need to seed it with a bottled bacteria product (Dr Tim's One & Only, Microbacter7 or Fritz Turbostart 900 all work).

1. Mix saltwater to 1.025 SG at 25 °C. Fill the tank.

2. Add rock and sand, run the return pump and heater.

3. Dose ammonia to 2 ppm using a pure ammonium chloride source (Dr Tim's Ammonium Chloride is the standard).

4. Add bottled bacteria according to the bottle. Lights off for the first week — algae loves light and you don't need it yet.

5. Test ammonia and nitrite daily. When both read 0 ppm within 24 hours of a fresh 2 ppm ammonia dose, the cycle is complete.

Ammonia: 0 ppm.

Nitrite: 0 ppm.

Nitrate: anything under 20 ppm — a small water change clears it.

pH: 8.1–8.4. Salinity: 1.024–1.026 SG. Temperature: 24–26 °C.

Log these in TankBase Tank Log so you can see the curve drop — it's the most satisfying graph you'll ever produce.

Adding fish before nitrite hits zero. The single biggest killer of new tanks.

Doing water changes mid-cycle. You're starving the bacteria of the food they need to grow.

Trusting a 'silent cycle'. Even with live rock, test for two clean readings in a row before adding anything alive.