Starting a reef aquarium is exciting, but one crucial step determines whether your corals and fish thrive or struggle: cycling the tank. Understanding reef tank cycling basics will help you build a stable, healthy ecosystem that can handle the demands of saltwater life.

What Is Reef Tank Cycling?

Cycling is the process of establishing beneficial bacteria that convert toxic waste into safer compounds. In a reef tank, this means building a strong biological filter that can handle fish waste, uneaten food, and other organics.

The nitrogen cycle has three main stages:

• Ammonia (NH₃): Produced from waste and decaying matter; highly toxic.
• Nitrite (NO₂^–): Formed as bacteria convert ammonia; still dangerous.
• Nitrate (NO₃^–): Final product; far less toxic, removed through water changes and export methods.

When your tank is fully cycled, ammonia and nitrite should read zero, and nitrate will be present at low to moderate levels. For more on long-term water quality, see our guide on reef tank water parameters.

How to Cycle a Reef Tank Step by Step

1. Set Up Rock, Sand, and Saltwater

Begin with quality live rock or dry rock, your chosen substrate, and properly mixed saltwater at the correct salinity (usually 1.025–1.026 specific gravity). Ensure your heater, powerheads, and filtration are running before you start the cycle.

2. Add an Ammonia Source

You need a controlled source of ammonia to feed the bacteria:

• Pure household ammonia (no surfactants or scents), or
• A small amount of fish food added daily.

Target an initial ammonia level of about 1–2 ppm. This is enough to grow a strong bacterial colony without overloading the system.

3. Test Regularly

Use a reliable saltwater test kit to track progress:

• Week 1–2: Ammonia rises, then starts to drop. Nitrite begins to appear.
• Week 2–4: Nitrite peaks, then gradually falls as nitrate increases.
• End of cycle: Ammonia and nitrite reach zero within 24 hours of dosing; nitrate is present.

Tip: Don’t rush this stage. A typical reef tank cycle takes 3–6 weeks, sometimes longer depending on rock type and temperature.

4. Perform a Large Water Change

Once ammonia and nitrite are consistently zero, perform a 25–50% water change to reduce nitrate and replenish trace elements. This is a good time to double-check salinity, temperature, and pH before adding any livestock.

5. Add Livestock Slowly

Start with a small, hardy clean-up crew rather than multiple fish at once. Give the system 1–2 weeks to adjust to each new addition. For stocking ideas and pacing, see our article on beginner reef fish stocking.

Common Cycling Mistakes to Avoid

• Adding fish too early: Livestock should never be used to “test” the cycle; it exposes them to toxic ammonia and nitrite.
• Over-cleaning filters and rock: Aggressive rinsing in tap water can kill beneficial bacteria.
• Chasing numbers daily: Test regularly, but expect fluctuations. Focus on the overall trend toward zero ammonia and nitrite.
• Overfeeding during the cycle: Too much organic matter can cause stubborn nitrate and algae issues later.

With patience and a bit of testing, reef tank cycling becomes a one-time investment in long-term stability. Once your biological filter is established, your focus can shift to fine-tuning nutrients, dialing in lighting, and planning your coral layout. For next steps, check out our overview of reef tank maintenance schedules to keep your new system running smoothly.

Take your time, trust the process, and your reef will reward you with a stable, vibrant display for years to come.

Sources

• Sprung, J. & Delbeek, J. C. The Reef Aquarium, Volume 1. Ricordea Publishing.
• Fenner, R. M. The Conscientious Marine Aquarist. TFH Publications.
• Holmes-Farley, R. (Reef chemistry articles and nitrogen cycle discussions).