How to cycle a saltwater tank (the right way)

Cycling is the single most important step before adding fish. It's also where most beginner reef tanks are quietly sabotaged — by impatience, by bad advice, or by both.

What "cycling" actually means

Cycling establishes two bacterial colonies in your live rock and substrate:

1. Ammonia oxidizers convert ammonia (toxic) → nitrite (still toxic).
2. Nitrite oxidizers convert nitrite → nitrate (much less toxic, exported via water changes or denitrification).

Until both colonies are established and at full population, anything you put in the tank that produces ammonia — fish, inverts, even uneaten food — will poison itself.

The honest timeline: 3–6 weeks

Forget anything that promises an "instant cycle." It is not instant.

• Days 0–3: Set up tank, mix saltwater to 1.025 SG, add live rock and sand. Heater on, powerheads running.
• Days 3–7: Add an ammonia source. Either a small amount of pure ammonium chloride (Dr. Tim's, Fritz Turbo Start, etc.) dosed to 2 ppm, or a piece of raw shrimp left in the tank.
• Days 7–21: Test daily. Ammonia rises, then falls. Nitrite rises as ammonia falls.
• Days 21–35: Nitrite falls. Nitrate begins to climb.
• End: You can dose 2 ppm ammonia and have it process to zero ammonia + zero nitrite within 24 hours. That is a cycled tank.

What to test (and what to ignore)

You need three test kits to cycle responsibly:

• Ammonia (NH₃/NH₄)
• Nitrite (NO₂)
• Nitrate (NO₃)

Ignore alkalinity, calcium, and magnesium for now — those matter once corals arrive, not during cycle.

Common mistakes

1. Adding fish on day 7. This is the classic killer. The cycle isn't done; fish die or are permanently weakened.
2. Using freshwater bacteria starters. Most "cycle in a bottle" products designed for freshwater don't work for saltwater. Buy marine-specific.
3. Doing water changes during the cycle. This removes the very ammonia your bacteria need to grow on. Don't.
4. Believing "live rock" is shortcut enough. Cured live rock can shorten cycling — but verify with the dosing test before adding livestock.

When the cycle is done

Run the verification test: dose ammonia to ~2 ppm. If it reads zero ammonia and zero nitrite within 24 hours, you can begin stocking. Start with one hardy fish (e.g. a captive-bred clownfish), wait 2 weeks, then add the next.

The patience required for cycling will calibrate the patience required for the rest of the hobby. Don't fight it.