Starting a saltwater aquarium is exciting, but before adding fish and corals, you need to learn how to cycle a reef tank correctly. A proper cycle builds a stable population of beneficial bacteria that convert toxic ammonia into less harmful nitrate, creating a safe environment for reef life.

What Is Cycling and Why Does It Matter?

Cycling is the process of establishing the nitrogen cycle in your new reef tank. When waste (food, fish waste, dead organisms) breaks down, it releases ammonia (NH₃), which is highly toxic. Beneficial bacteria convert ammonia to nitrite (NO₂^–), then other bacteria convert nitrite into nitrate (NO₃^–).

In a fully cycled reef tank:

• Ammonia stays at 0 ppm
• Nitrite stays at 0 ppm
• Nitrate rises slowly and is controlled with water changes and export methods

Skipping or rushing the cycle is one of the fastest ways to lose new livestock. Patience here saves money, time, and frustration later.

Step-by-Step: How To Cycle a Reef Tank

1. Set Up the Tank and Equipment

Before you start cycling, make sure your reef system is fully assembled:

• Display tank, sump (if used), and plumbing are leak-free
• Saltwater mixed to 1.025–1.026 specific gravity
• Heater set to 77–79°F (25–26°C)
• Return pump and powerheads running for good circulation
• Rock and sand in place (live rock or dry rock, aragonite sand)

For more on getting started, see our guide on setting up a reef tank.

2. Add an Ammonia Source

Your beneficial bacteria need a food source. There are three common ways to supply ammonia:

• Pure ammonia (fishless cycling): Add unscented household ammonia and test until you reach ~2 ppm.
• Raw shrimp: Place a small piece of raw shrimp in a mesh bag in the tank and let it decompose.
• Fish food: Add a pinch of food daily to rot and produce ammonia.

Tip: Fishless cycling with pure ammonia gives you the most control and avoids stressing live animals.

3. Seed Beneficial Bacteria

While bacteria will appear naturally, seeding speeds things up and makes the cycle more predictable. You can use:

• Bottled nitrifying bacteria products
• A small piece of live rock or a cup of live sand from an established, healthy tank

Follow the dosage instructions for any bottled bacteria you choose, and keep the system running with stable temperature and salinity.

4. Test Regularly and Track the Cycle

Use reliable test kits for ammonia, nitrite, and nitrate. A typical pattern looks like this:

• Days 1–7: Ammonia rises, nitrite 0, nitrate 0
• Days 7–21: Ammonia falls, nitrite rises, nitrate begins to appear
• Days 21–35+: Ammonia 0, nitrite 0, nitrate rising

Your tank is considered cycled when you can dose ammonia to ~2 ppm and see both ammonia and nitrite return to 0 ppm within 24 hours. At this stage, perform a large water change (25–50%) to bring nitrates to a reasonable level.

After the Cycle: Adding Livestock Safely

Once your reef tank is cycled, resist the temptation to fully stock it right away. Instead:

• Add only 1–2 hardy fish at first, or a small cleanup crew
• Test weekly for ammonia and nitrite after each new addition
• Increase bioload slowly over several weeks

When you’re ready, you can move on to planning your first corals. Check out our article on best beginner reef fish and our reef tank maintenance schedule to build a long-term plan.

Learning how to cycle a reef tank is the foundation of a thriving marine system. By providing an ammonia source, seeding beneficial bacteria, and patiently testing until the tank processes waste efficiently, you set the stage for healthy fish and corals for years to come.

Sources

• Spotte, S. (1992). Captive Seawater Fishes: Science and Technology. Wiley-Interscience.
• Fenner, R. (2001). The Conscientious Marine Aquarist. Microcosm.
• Delbeek, J.C., & Sprung, J. (1994–2005). The Reef Aquarium Vol. 1–3. Ricordea Publishing.