Fast Aquatics

Short answer

4-6 weeks for a typical fishless cycle using ammonia drops. Saltwater tanks with established live rock can cycle in 2-3 weeks. Adding live bacteria starters (Dr. Tim's One and Only, Fritz Zyme 7) shortens the timeline by 30-50%. Never add fish to a tank that hasn't completed a cycle - it's the #1 cause of new-tank fish loss.

In depth

Cycling means establishing the bacterial colonies that convert toxic ammonia (from fish waste, decaying food) into nitrite (still toxic) and finally to nitrate (relatively safe at low levels). The colonies grow on every surface in the tank - filter media, substrate, decor, glass walls.

Standard fishless cycle (recommended for all new tanks)

  1. Set up the tank fully: substrate, hardscape, filter, heater, lights. Run for 24-48 hours to ensure equipment works and parameters are stable.
  2. Dose ammonia to 2-3 ppm using pure household ammonia (no surfactants - check the bottle). Test with an ammonia kit (API or Salifert).
  3. Wait 7-14 days. Ammonia stays at 2-3 ppm initially as bacteria establish. Test daily.
  4. Nitrite spike: ammonia drops, nitrite rises. This phase lasts 5-14 days. Re-dose ammonia to maintain 2-3 ppm if it falls below.
  5. Nitrite drop, nitrate rise: nitrite falls to 0, nitrate rises. Cycle is complete when both ammonia and nitrite read 0 within 24 hours of dosing 2 ppm ammonia.
  6. Large water change (75%+) to lower nitrate to 5-20 ppm before adding fish.

Bacteria-starter cycle (2-3 weeks)

Adding bottled live nitrifying bacteria (Dr. Tim's One and Only is the gold standard) shortcuts the establishment phase. Dose per package directions, then dose ammonia to 2 ppm. Most tanks complete the cycle in 14-21 days using this method.

Cycling with established live rock (saltwater only)

Live rock from an established system already has bacterial colonies. A 60-75% live rock setup can cycle in 5-10 days. The cycle still happens (bacteria die during transit and need to re-establish) but it's dramatically faster than starting from scratch.

What kills cycles

  • Antibiotics or antiparasitic medications - they kill the bacterial colonies you're trying to establish
  • UV sterilizers running during the cycle - reduces free-swimming bacteria
  • Carbon filtration - removes ammonia before bacteria can use it
  • Excessive water changes - dilutes the ammonia food source

More questions

Can I cycle with fish in the tank?

Yes but it's harder on the fish - they suffer ammonia and nitrite exposure during the establishment phases. If you must, use only the hardiest species (cherry shrimp, white cloud minnows, or zebra danios), keep stocking minimal, and do daily water changes.

Why is my cycle stuck at nitrite?

Nitrite-converting bacteria (Nitrobacter) are slower to establish than ammonia-converters (Nitrosomonas). The "stuck" feeling is normal - just keep dosing ammonia to 2 ppm and wait. Adding more bottled bacteria can speed it up.