How to Cycle a Saltwater Tank: Tips To Help You Succeed with Your New Aquarium

Setting up a new saltwater aquarium is an exciting project. If you’re like us, we can’t wait to start building up the reef by adding fish, corals and other invertebrates, but, a newly set-up aquarium is not biologically mature enough to handle a full load of reef livestock. 

New tanks need time to build a stable biological foundation.  Aquarists refer to the this maturation time as a “cycle.”

Phantom feeding a new aquarium during the cycle.

Phantom feeding a new aquarium during the cycle.

When cycling a new tank, you are essentially growing the bacteria that are the driving force behind the nitrogen cycle. These bacteria will process organic waste in your aquarium making it safe for fish and other tank inhabitants. Without this bacterial foundation, your aquarium will not survive.

An established reef aquarium contains millions of microbes that work 24-7. These microscopic bacteria work to eliminate waste products like ammonia, break down organic matter and recycle nutrients.

Bacteria on Live Rock

Bacteria on Live Rock

These microbe-driven processes are often referred to as the “biological filtration” inside your tank and can be comprised of a wide variety of bacteria species based on the conditions in your tank.

Nitrosomonas, Nitrobacter, and Nitrospira are all nitrifying bacteria types that can be found in your aquariums alongside a ton of others. The important fact to understand here is that every tank is different and the bacteria types that exist and dominate your aquarium will be unique.

Atlantic Blue Tang in an aquarium.

Atlantic Blue Tang in an aquarium.

Fish excrete toxic ammonia directly from their gills but Ammonia also comes from organic matter breaking down inside the aquarium. When solid fish & invert waste or uneaten food decompose, it will release toxic ammonia into the water.

Thanks to the nitrogen cycle and the bacteria that drive it, this toxic Ammonia does not build up in the aquarium.

Diagram of the Nitrogen Cycle

Diagram of the Nitrogen Cycle

The nitrogen cycle is a two-step process involving two types of nitrifying bacteria.

In the first step, ammonia is converted to nitrite by what scientists call ammonia oxidizing bacteria or AOB for short. This is simply a classification we use to group together the various species of bacteria that convert ammonia to nitrite.

Next, nitrite oxidizing bacteria or N.O.B. for short, converts nitrite to nitrate which is far less toxic to fish and other animals when compared to ammonia or nitrite.

Nitrate is only harmful when it starts to stack up inside your aquarium which is exactly why some of us do water changes. Water changes remove nitrate that would otherwise just keep stacking up.

This is a simplified explanation of what happens in a mature or “cycled” aquarium and why you must give it time before adding your livestock. New tanks need to go through a period of maturation during the development of this nitrifying bacteria.

Timeline of the Nitrogen Cycle

Timeline of the Nitrogen Cycle

To track the process you can utilize some test kits and test the water for Ammonia, nitrite and finally nitrate.  First, you will see the ammonia level rise and then start to fall when nitrites become present. Finally you will see a rise in nitrates and the ammonia and nitrite levels will be zero, this means the cycle has completed and both types of bacteria are present in your aquarium.

There are a couple of things you need to do in order to get the cycle started inside a new tank. Seed the tank with bacteria and provide a source of ammonia to feed that bacteria.  

There are several ways to “seed” a brand new tank with bacteria. You can add a few pieces of seasoned live rock or live sand from an established aquarium.  

Seeding a new aquarium

Seeding a new aquarium

The downside here is you run the risk of introducing pests. But, cycling an aquarium with seasoned live rock is still a widely- practiced and very common method.

If using live rock or sand, you only need a little bit to seed the aquarium with bacteria.  It is perfectly suitable to fill your tank with DRY rock and sand then add a small piece of LIVE rock or small scoop of sand to introduce the bacteria.  As long as you provide an ammonia source, this bacteria will soon grow and populate onto all of the surfaces in your tank, including the dry rock and sand.

Another method of seeding the tank is using a liquid bacteria supplement.

Not all liquid bacteria products are made equal. Some formulations contain microbes and enzymes that will take time to grow and become abundant in an aquarium, such as Brightwell Aquatics MicroBacter7. This is one of our favorites.

Live nitrifying bacteria

Live nitrifying bacteria

If you wish to really jump start the cycle process, you might look at Dr. Tim’s One and Only or Fritz Aquatics TurboStart because these contain live nitrifying bacteria that will immediately go to work in your tank. These product will work  so long as ammonia is present, and these products can help to shorten the length of time it takes to make the aquarium safe for the addition of fish.

No matter which brand of bio-additive you choose, be sure to follow the manufacturer’s instructions closely. Each of the instructions are are indeed different and will contain different concentrations and species of microbes.

Methods of adding bacteria

Methods of adding bacteria

One thing to note is that it’s not unusual for aquarists to use both liquid nitrifiers as well as live rock and sand.  This is because you really want to do everything you can to start a diverse and healthy population of bacteria in the aquarium as quickly as possible.  Get bacteria from various sources and larger amounts will only help the process.

Yellowtail Damselfish (Chrysiptera parasema)

Yellowtail Damselfish (Chrysiptera parasema)

After you added live bacteria to the tank, you’ll need a source of ammonia.

Some aquarists add a live fish and let it produce ammonia naturally. Feed the fish sparingly and monitor the ammonia and nitrite. Levels should moderately rise and fall as discussed earlier, indicating the aquarium has cycled.  This can however, put that fish at risk of ammonia or nitrite poisoning.

The “fishless cycle” is designed to grow-out the bio filter before marine life is added. This method relies on a higher initial ammonia dose to “force feed” the bacteria and develop a substantial biofilter before adding livestock.

Fish-less cycle

Fish-less cycle

Some aquarists drop in a piece of shrimp or phantom feed fish food. As the food decays, ammonia is released.

Another method involves adding a prepared ammonia solution to the tank. Dr. Tim’s Ammonium Chloride is designed specifically for this purpose and should be used alongside a Nitrifying bacteria product such as Dr. Tim’s One and Only.

Timeline of cycling a new aquarium

Timeline of cycling a new aquarium

It can take around 6 weeks for the aquarium to complete the cycle, depending on the vitality of the seed bacteria. Nitrifying bacteria are relatively slow growers. They respond to ammonia and nitrite by dividing and forming larger colonies.

Plan on four to six weeks, especially if you’re going with a fish-less cycle. The key is to be patient!

Just let the bacteria do their thing. You’ll be able to track the process with test kits.

It is important to remember that when you add more livestock, the bio filter has to adjust and grow to meet the higher ammonia levels produced by the new marine life. Keep an eye on ammonia and nitrite levels every time you add more aquatic life. Never add a large amount of fish at one time.

I typically recommend 1-2 fish every other week after you have gone through a successful cycle to ensure you do not overload the aquarium with nutrients. This method gives it plenty of time for the bacteria to establish and keep up with the additional waste in the aquarium.

Brand new aquarium, waiting for new fish and corals.

Brand new aquarium, waiting for new fish and corals.

In an age of instant-gratification, some hobbyists become worried that the tank won’t cycle or just become impatient and start adding animals too quickly.

Take it from me and the thousands of people keeping aquariums before me, don’t be that guy.  It will only lead to killing animals and getting frustrated. Just set up the tank, add your bacteria and ammonia source then leave it alone for a couple weeks before you start monitoring the waste parameters.

There’s really no secret to cycling a new tank. Aquarists have been starting up marine aquariums for hundreds of years, without even knowing about the nitrogen cycle. Just trust the process, test your water and your tank will be ready for fish and corals in no time.

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