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Setting up a new reef aquarium is an exciting journey, but patience is key! There’s an old Reefing adage that says, “Nothing good happens quickly in an aquarium,” so be sure to temper your expectations and allow your aquarium plenty of time to cycle.
Before adding your first fish or coral, it’s essential to establish a stable bacterial ecosystem. This process, called cycling, builds up beneficial nitrifying bacteria that break down waste like toxic ammonia and nitrite to keep water conditions safe for marine life. A properly cycled tank is non-negotiable for your fish.
At The Biota Group, we believe in responsible reefkeeping that prioritizes sustainability and long-term success. Let’s walk through the best way to cycle your saltwater aquarium while keeping both science and sustainability in mind.
What Is the Nitrogen Cycle and Why Is It Important?
The nitrogen cycle is nature’s way of processing waste in aquatic environments. In simple terms:
Ammonia (NH3) is produced from fish waste, uneaten food, and decaying organic matter. Ammonia is highly toxic to marine life.
Nitrite (NO2-) is formed when beneficial bacteria, called Nitrosomonas, break down ammonia. Nitrite is still toxic, but slightly less so than ammonia.
Nitrate (NO3-) is created when another type of bacteria, Nitrobacter, converts nitrites into nitrates. Nitrate is far less harmful and can be removed through water changes or absorbed by macroalgae and corals.
A fully cycled tank has enough beneficial bacteria to efficiently convert ammonia into nitrate, creating a balanced ecosystem where marine life can thrive. A lot of new aquarists tend to think of their tank as “not cycled” or “cycled,” but the reality is that the beneficial bacteria is, in general, proportional to the bioload, or amount of ammonia produced in your aquarium. This is why adding a lot of livestock at once or overfeeding can overwhelm the good bacteria, even in a cycled aquarium.
Step-by-Step Guide to Cycling Your Reef Tank
1. Set Up Your Aquarium with Sustainable Live Rock & Sand
Adding live rock and live sand helps jumpstart your cycle by introducing beneficial bacteria to your tank. At Biota, we encourage using aquacultured live rock rather than wild-collected rock. Not only is cultured live rock better for the environment, it is also beneficial to your reef because it is far less likely to carry pests like Aiptasia, Majano, coral-eating crabs, Mantis shrimp, etc. Many companies that culture live rock add only beneficial microfauna like copepods and amphipods along with nitrifying bacteria to their rock.
Dry rock is another option if you ensure you’re not introducing pests to your tank via live rock. Another benefit of dry rock is that it’s the least expensive option. If using dry rock, cycling will take longer since bacteria must colonize from scratch.
Another good option is to buy live rock from a local aquarist who is rehoming their aquarium and its contents. Check with your local aquarium club, and you might find a great deal on cycled live rock . Be sure to inspect it well for pests and excess algae before purchasing.
2. Start the Cycle with an Ammonia Source
Beneficial bacteria need an ammonia source to kickstart the nitrogen cycle. You can introduce ammonia in one of three ways:
Fishless Cycling
Add pure Ammonia to start the cycle. Purity is of utmost importance, and many ammonia products contain toxic surfactants. Dr. Tim’s Ammonium Chloride is a great option because it is aquarium safe, and it has detailed instructions on dosing and testing during the cycle.
small amount of fish food and let it decompose.
Live Rock Cycling (Not Recommended)
If you’re using natural live rock, die-off from microorganisms on the rock may produce enough ammonia to start the cycle.
Adding Hardy Fish (Not Recommended)
Some people cycle tanks with hardy fish, but we strongly advise against this, as it exposes them to toxic ammonia and nitrite levels, causing unnecessary stress, permanent damage, or death.
3. Test Water Parameters Regularly
During the cycle, regularly test for:
Ammonia – Spikes first, then drops as bacteria establish.
Nitrite – Increases after ammonia and then declines.
Nitrate – Rises as nitrites are converted.
Using a high-quality saltwater test kit ensures you monitor progress accurately.
4. Seed the Tank with Beneficial Bacteria
To speed up cycling, you can add a bacteria supplement like Dr. Tim’s One & Only. This introduces live nitrifying bacteria to your tank and can slightly reduce cycling time.
5. Wait for the Cycle to Complete
Cycling takes 6-8 weeks, depending on your method and bacteria growth. You know your tank is cycling when the ammonia peaks, then falls while the nitrite peaks. After the nitrite peaks and falls while nitrate increases, keep consistently checking the parameters to make sure they’re stable. Your tank is ready to add a hardy fish when your ammonia and nitrite are consistently zero.
6. Perform a Water Change & Add Your First Livestock
Once the cycle is complete, do a 20–30% water change to lower nitrates and remove any lingering waste. When doing water changes, ensure that your new water has the same pH, salinity, and temperature as the water in your tank to prevent shocking the biological filtration. Now, you can introduce your first captive-bred fish!
At Biota, we recommend starting with captive-bred fish like Orange Clownfish or Biota Tangs. They adapt better to aquariums, reducing stress and disease risk compared to wild-caught fish.
FAQs About Cycling a Reef Aquarium
Can I Cycle My Tank with Corals?
Hardy soft corals like Mushroom corals or Zoanthids can sometimes be introduced during the later stages of the cycle, but it’s best to wait until water parameters stabilize.
How Do I Know If My Tank Crashed?
If ammonia or nitrites suddenly spike after stabilizing, it could be due to overfeeding, a die-off event, taxing the bioload by adding too much livestock at once, or removing too much beneficial bacteria during cleaning. Keep an eye on water tests and make adjustments as needed.
Do I Need a Protein Skimmer During Cycling?
Not necessarily. A skimmer helps remove organic waste, but during cycling, it’s okay to let nutrients build up to encourage bacterial growth. Once livestock is added, a skimmer becomes more beneficial.
What are some common mistakes new aquarists make when cycling a new reef aquarium?
1. Rushing the Cycle
Many beginners get impatient and try to add fish or corals too soon. The nitrogen cycle takes time, so adding livestock too early can lead to ammonia spikes and fatalities. Keep in mind that the bacteria that convert nitrite to nitrate grow more slowly than the bacteria that convert ammonia to nitrite. Skipping this step leads to problems.
2. Not Testing Water Parameters Regularly
Failing to monitor ammonia, nitrite, and nitrate levels means you won’t know when the cycle is complete. Invest in a reliable test kit and track your water chemistry to ensure safe conditions before adding livestock.
3. Not Using a Proper Ammonia Source
The cycle needs an ammonia source to kickstart bacterial growth. Some people add fish too early (which is harmful to them), while others don’t add enough ammonia for bacteria to establish. Fishless cycling using bottled ammonia is a better option, and bottled ammonia like Dr. Tim's Ammonium Chloride include detailed instructions on how much ammonia to dose and when.
4. Relying Too Much on Bacteria in a Bottle
While bottled bacteria products can help jumpstart the process, they don’t instantly complete the cycle. Some aquarists assume adding these means the tank is immediately ready for fish - this isn’t true! You still need to monitor parameters and allow the bacteria to establish naturally.
5. Doing Large Water Changes Too Early
Water changes are essential, but if done too soon, they can slow the cycling process by removing beneficial bacteria before they’ve colonized your tank. Wait until ammonia and nitrite levels drop before performing a water change. You may notice your water is cloudy during the cycle, but this is normal and expected, and what you're seeing is a bacterial bloom.
6. Using Dry Rock Without Seeding It
Dry rock and sand won’t have any beneficial bacteria, making cycling take much longer. Seeding with live rock, live sand, or a bacteria supplement can speed up the process.
7. Overcomplicating the Process
Some aquarists use multiple cycling methods at once (bacteria in a bottle, ghost feeding, ammonia dosing) without a clear strategy. Keep it simple - choose a reliable method and stick to it.
8. Adding Livestock Before Testing Stability
Even if ammonia and nitrite are at zero, your system isn’t ready until it can consistently handle small bio-load increases. Start with hardy corals and fish and continue testing water parameters before adding more.
9. Adding Too Much Livestock at Once
When you increase the bio-load too quickly, the ammonia-eating bacteria colony isn't established or large enough to handle the amount of ammonia the new livestock are producing. This can lead to die-offs.
10. Adding Too Much Ammonia or a Heavy Bio-load
If the ammonia reaches an excessively high level, it can actually hinder the growth of ammonia-eating bacteria. Ammonia-eating bacteria (primarily Nitrosomonas species) thrive within a certain ammonia range, but their growth slows or stops at extreme concentrations. Here’s what happens at different levels:
0.25–5.0 ppm: Ideal for bacterial growth
This range allows beneficial bacteria to thrive and establish a stable colony.
Above 5.0 ppm: Growth slows down significantly
High ammonia levels can become toxic even to the bacteria that consume it.
Above 10.0–15.0 ppm: Bacteria can go dormant or die
Extremely high ammonia levels inhibit bacterial enzyme function, halting the cycling process. If ammonia levels exceed 5.0 ppm during cycling, a partial water change can help keep conditions optimal for bacterial colonization.
When ammonia levels increase, pH naturally decreases. Toxic ammonia is converted to less toxic ammonium when pH levels are lower. If you have fish in your tank already and have an ammonia spike, use caution when doing water changes. The higher pH in the new water can increase the toxicity of ammonia in the tank and harm the fish. That's why it's so important to cycle your aquarium before you add fish and to add small numbers of fish in the beginning. If necessary, you can use an ammonia detoxifier like Amquel + to reduce the stress of toxic ammonia on your fish in this case.
Patience Leads to Success
Cycling your tank is the foundation for a thriving reef aquarium, and patience is key. By taking the time to establish a stable nitrogen cycle, you’ll create a healthy environment where fish, corals, and invertebrates can flourish.
Consider the size and age of your aquarium when stocking with new fish.
Ready to stock your newly cycled tank? Check out our sustainably aquacultured fish, corals, and inverts at shop.thebiotagroup.com!
