To cycle a reef tank, add a steady source of ammonia (fish food, pure ammonia or a cycling product), let bacteria grow to process it, and test ammonia and nitrite regularly. The cycle is done when both read zero and nitrate is present. Do a water change to lower nitrate, then add a single hardy fish. Without bottled bacteria this usually takes 3–6 weeks.
What cycling does
Bulk Reef Supply explains the nitrogen cycle: fish waste and food release ammonia, which is very toxic. One group of bacteria turns ammonia into nitrite, still toxic. A second group turns nitrite into nitrate, which is much less harmful. Cycling grows enough of both groups before livestock arrives.
Step by step
Bulk Reef Supply’s three steps, with the testing that goes with them.
- Add an ammonia source. Fish food, pure ammonia or a cycling product. Keep it steady.
- Add bacteria, if you like. Bottled bacteria with live cultures can speed things up. Follow the label dose; Bulk Reef Supply notes underdosing causes more problems than a little extra.
- Test. Ammonia and nitrite every two or three days, nitrate weekly. Log each one.
- Wait for the zeros. Bulk Reef Supply’s cycling guide looks for ammonia at zero for two or three weeks running, nitrite at zero and nitrate present.
- Water change. Lower the nitrate that built up, then add one small, hardy fish.
What the readings do
In a typical cycle, ammonia rises first, then falls as nitrite rises. Nitrite then falls as nitrate rises. Logging every test lets you see where you are on that curve. See ammonia and nitrite during the cycle.
Diatoms and lights
A brown film on the sand and glass is common during or just after cycling. It’s diatoms, and it passes. Many keepers run the lights on a short schedule during the cycle to limit early algae.
After the cycle
Bulk Reef Supply suggests starting with one small, hardy fish, then adding one at a time, at least two weeks apart. Adding too many at once risks an ammonia spike that sets the bacteria back. See when to add your first fish.
Common questions
How do I know my reef tank has cycled?
Ammonia and nitrite read zero and nitrate is present. Bulk Reef Supply’s guide looks for zero ammonia for two or three weeks.
Does bottled bacteria really work?
Products with live cultures can shorten the cycle. Still add livestock slowly and keep testing.
Can I cycle with live rock?
Live rock brings bacteria and often speeds things up. Test the same way.
Sources and limits
Last checked 13 October 2026. This guide brings together the published advice below. Product instructions come first, and an experienced local shop or club can help with anything specific to your tank.
- Bulk Reef Supply: the nitrogen cycle in your aquarium
- Bulk Reef Supply: cycling your saltwater aquarium