How to Cycle a Reef Tank: Complete Cycling Guide (Dry Rock, Live Rock, Bottled Bacteria)

Cycling a reef tank means growing the bacteria that turn fish waste into safer compounds. Ammonia (toxic) becomes nitrite (also toxic) becomes nitrate (much less toxic). It takes 2 to 6 weeks and cannot be safely rushed.

Guides January 12, 2026 By Joe Marino 2 min read

What "cycling" actually means

When a fish or coral pees, breathes, or dies, it releases ammonia. Ammonia at any measurable level is toxic to almost everything. A healthy reef has two bacterial colonies: one that converts ammonia to nitrite, another that converts nitrite to nitrate. Cycling means growing those colonies until they can keep up with the bioload you plan to add.

Three ways to cycle

Dry rock + bottled bacteria + ammonia source: cleanest, slowest (4–6 weeks). No risk of importing pests.
Cured live rock from a vendor: fast (1–2 weeks) because bacteria are already present, but may import small pests (bristle worms, micro-stars, sometimes worse).
Mix: most dry rock plus a small piece of live rock as a seed. Speeds dry rock by 1–2 weeks and limits pest risk.

The cycling timeline (with bottled bacteria + dry rock)

Day 1: Tank filled, salinity at 1.025, temperature stable at 78°F. Add bottled bacteria (Dr Tim's One and Only, Brightwell MicroBacter7, Fritz TurboStart 900). Add ammonia source: 2 ppm dose of pure ammonia, or a small piece of raw cocktail shrimp.
Days 1–14: Ammonia rises, peaks around day 3–7, then starts dropping as ammonia-eating bacteria multiply. Nitrite appears as ammonia falls.
Days 14–28: Ammonia hits zero. Nitrite climbs, peaks, then falls as nitrite-eating bacteria catch up.
Days 28–42: Nitrite hits zero. Nitrate is detectable (typically 5–25 ppm). The cycle is done.
Test daily for the first 2 weeks, every 2 days after. Reef Trak's parameter tracking with custom alerts makes this easy.

How to know the cycle is finished

When you can dose 2 ppm of ammonia and both ammonia and nitrite read zero within 24 hours, the cycle is complete. At that point, do a 20% water change to drop nitrate, and add a single hardy cleanup-crew animal as a small bioload test.

Frequently asked

How long does it take to cycle a reef tank?

2 to 6 weeks depending on method. Cured live rock cycles fastest (1–2 weeks). Dry rock with bottled bacteria cycles slowest (4–6 weeks) but is cleanest.

Can I cycle a reef tank in a week?

Only with fully cured live rock from a vendor that already has active bacteria. Even then, test daily for at least a week to confirm ammonia and nitrite are zero before adding livestock.

What ammonia source should I use to cycle?

Dr Tim's Ammonium Chloride dosed to 2 ppm is the controlled option. A small piece of raw cocktail shrimp works too but is messier. Bottled bacteria seeds the colony in either case.

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