Cycling a reef tank is the essential process of establishing a healthy biological filter, which relies on beneficial bacteria to convert toxic waste products like ammonia and nitrite into less harmful nitrate. This biological filter is crucial for the survival of fish, corals, and other inhabitants.
What is Tank Cycling?
Tank cycling, specifically the nitrogen cycle in aquariums, involves cultivating colonies of specific bacteria. These bacteria live on surfaces within the tank, such as live rock, dry rock, sand, and filter media.
The process follows these steps:
- Ammonia Production: Waste from fish, uneaten food, and decaying organic matter breaks down into ammonia (NH₃), which is highly toxic.
- Nitrite Conversion: One type of bacteria (Nitrosomonas species) consumes ammonia and produces nitrite (NO₂⁻), which is also very toxic.
- Nitrate Conversion: Another type of bacteria (Nitrobacter species) consumes nitrite and produces nitrate (NO₃⁻), which is much less toxic.
- Nitrate Removal: Nitrate accumulates in the tank and is typically removed through water changes, or by specific methods like using macroalgae, refugiums, or denitrification reactors.
Cycling is complete when these bacterial colonies are large enough to process all the ammonia and nitrite produced by the tank's future inhabitants, resulting in zero ammonia and zero nitrite readings.
Methods for Cycling a Reef Tank
There are several ways to initiate the cycling process, each with its own advantages and timeline.
1. Traditional Method: Using Live Rock
As stated in the reference, the "traditional" and most common way of cycling a reef tank is to use live rock.
- Process: Just add live rock (and sand, which is optional) to your display tank and/or sump, fill with saltwater, and wait.
- Why it works: Live rock is rock collected from the ocean that is teeming with beneficial bacteria. These bacteria, along with other marine organisms, help kickstart the nitrogen cycle.
- Considerations: The reference notes that En route to your home, the rock spends several days out of water. This die-off of organisms on the rock creates ammonia, which helps feed the initial bacterial colonies. However, significant die-off can cause a large ammonia spike, potentially requiring extra water changes.
- Timeline: This method can take anywhere from 2 to 8 weeks, depending on the amount and quality of the live rock.
2. Using Dry Rock and Ammonia
This is another very common method, often used with modern aquascaping that utilizes "dry" or "aquacultured" rock.
- Process: Fill the tank with saltwater containing dry rock and sand. Add a source of ammonia (like pure liquid ammonia) to the water daily to reach a specific concentration (e.g., 2-4 ppm).
- Why it works: Adding ammonia provides the food source necessary for bacteria to colonize the dry rock surfaces and build up their numbers.
- Considerations: Requires careful monitoring and consistent ammonia dosing. Can take slightly longer than using live rock with minimal die-off.
- Timeline: Typically takes 4 to 8 weeks.
3. Using Bottled Bacteria
Specific commercial products contain live or dormant beneficial bacteria cultures that can be added to the tank water.
- Process: Set up the tank with rock and sand, fill with saltwater, and add the recommended dose of the bottled bacteria product. Some products recommend adding a small amount of ammonia or a food source as well.
- Why it works: Directly introduces the necessary bacteria to the tank, potentially speeding up colonization.
- Considerations: The effectiveness varies between products. It's still crucial to monitor parameters to confirm the cycle is complete.
- Timeline: Can potentially shorten the cycle to 1-3 weeks in some cases, but results vary.
Monitoring the Cycling Process
Regardless of the method used, monitoring water parameters is essential to track the progress of the cycle and determine when it's complete. Use a reliable saltwater test kit (liquid kits are generally more accurate than test strips).
Here's a typical progression of water parameters during a standard cycle:
| Parameter | Early Cycle (Weeks 1-2) | Mid Cycle (Weeks 2-4) | Late Cycle (Weeks 4+) | Cycle Complete |
|---|---|---|---|---|
| Ammonia (NH₃) | Spikes high | Decreasing | Near zero | 0 ppm |
| Nitrite (NO₂⁻) | Starts low, then spikes | High | Decreasing | 0 ppm |
| Nitrate (NO₃⁻) | Low/Zero | Starts increasing | Increasing | Detectable |
Signs the Cycle is Complete:
- Ammonia tests read 0 ppm.
- Nitrite tests read 0 ppm.
- Nitrate is detectable and likely increasing.
- Optional: You can confirm the cycle by adding a small amount of ammonia (e.g., enough to reach 1-2 ppm) and testing again 24 hours later. If both ammonia and nitrite are back to 0 ppm, the cycle is complete.
Once the cycle is complete, you can begin slowly stocking the tank with livestock. It's best to add a few hardy inhabitants first and gradually increase the bioload over several weeks.
