Cycling establishes the bacteria that convert toxic ammonia to nitrite and then to nitrate. A fishless cycle is safer and clearer than using fish as the ammonia source. Full science: nitrogen cycle guide.
Basic fishless cycle
- Fill with RO/DI mixed saltwater at 35 ppt, heat to 78–80 °F (25–27 °C), run pumps and lights on a short schedule.
- Add cured rock (or seed dry rock with live rock rubble or bottled bacteria).
- Dose ammonia to about 2 ppm (ammonium chloride). Avoid "invisible cycle" products that claim no testing.
- Test ammonia, nitrite and nitrate every 1–2 days.
- When 2 ppm ammonia drops to 0 ammonia and 0 nitrite within 24 hours, and nitrate is present, the cycle is functionally done.
- Do a large water change to bring nitrate down before adding livestock.
Typical timeline with dry rock and bottled bacteria: 2–6 weeks. Bare dry rock alone can take longer.
What to test during the cycle
- Ammonia and nitrite: the cycle markers
- Salinity and temperature: keep stable
- Alkalinity: can drift; correct gently if it falls below 7 dKH
Don't add corals until ammonia and nitrite are zero. Softies are more forgiving of early nutrient swings than SPS.
Sources
FAQ
Can I cycle with fish? Possible but stressful and outdated. Fishless is preferred.
Do I need a deep sand bed to cycle? No. Surface biofilms and rock pores are enough for nitrification.