The Salty Side: Saltwater Aquariums for Beginners

How to Cycle a Saltwater Aquarium

The very first process you will be introduced to when starting up a reef tank is something referred to as tank cycling. You will need to know how to cycle a saltwater tank. This is not something unique to marine keeping. The cycling process is also established in freshwater tropical tanks.

In fact, the tank cycling process is evident in every body of water on earth. It is a continual process that cycles through ammonia and nitrite and into nitrate, then nitrogen. It is known as the Nitrogen Cycle

saltwater aquarium live rock
Live rock encrusted with coralline algae and sponges

How does the nitrogen cycle work in a saltwater aquarium?

Tank cycling is more accurately described as the nitrogen cycle. It is an established, revolving, continuous process.  Once it is established in your tank, it will continue to ensure that your inhabitants can live comfortably in an enclosed environment.

With the aim of making this as informative as possible, I’ll introduce you to these helpful bacteria.

  • Nitrosomanas are nitrifying bacteria that live for the sole purpose of converting Ammonia (NH3) into Nitrite (NO2).


  • Nitrobacter are nitrifying bacteria that convert Nitrite (NO3) into Nitrate (NO2).

For the nitrogen cycle to kick off, both nitrosomanas and nitrobacter bacteria have to be present, even in small amounts.  

ammonia and nitrite toxic


Traditional Tank Cycling

Until several years ago, this method of tank cycling was the only way to establish a saltwater aquarium. Initially the process involved setting a tank up with dry ocean rock (also known as aragonite).

Ammonia was introduced to the tank by adding a prawn, or a muscle, or even ammonia and patiently waiting for nitrifying bacteria to populate the tank.

Through testing the water you could establish at what stage the nitrifying bacteria where at. If a test came back with high ammonia, then you knew that the nitrosomanas bacteria had not yet successfully established themselves.

When ammonia was diminishing or gone altogether, but nitrite was evident, the nitrosomanas bacteria where pretty well established, converting ammonia into nitrites.

When nitrates were being detected, you were confident that the nitrobacter bacteria were now converting nitrites to nitrate.

When no ammonia or nitrite was detected, you could slowly begin adding fish.

This process was known to take up to two months to establish the nitrifying bacteria required to correctly deal with future bioload.  Once your tank was cycled your rock could be called ‘live’.

Cycling a saltwater tank with live rock

Live rock , also aragonite, became more readily available. It brought huge changes to tank cycling. It was shipped from abroad after having spent time in the ocean*, and was alive with marine microorganisms and bacteria.

During shipment it was kept damp, and when it reached its country of destination it was put into holding tanks. Depending on the quality of the live rock it would include all sorts of hitchhikers directly from the sea of its origin.

The key with this live rock is that it already had an abundance of nitrifying bacteria living on and in it.

When live rock was purchased, the time it took to transfer it from its holding tank at your LFS to your aquarium determined how quickly a nitrogen cycle could be established in your tank.  The longer the period that live rock is kept out of the water, the more organisms and bacteria perish. This is referred to as ‘die-off’, which I’ll explain in more detail below.

How long should a saltwater tank cycle before adding fish?

This is the most common question asked by newcomers into the reefing world, and the answers can be a little confusing because their are two opposing camps on the subject.

Camp one is old school. Cycle a tank the old fashioned way by introducing a source of ammonia such as a shop bought shrimp that will decay over a period of weeks. After six to eight weeks bacteria will have established themselves on your rock and in your sand. This is neither the right way nor the wrong way. You achieve your end result of eventually adding fish after a month and a half (as long as you have no ammonia and nitrite present).

Camp two is a little more progressive. This camp like to move things along a little quicker and they have the tools to do just that. With the advent of bacteria in a bottle, such as ATM Colony. Simply put, the bacteria that camp one slowly propagate over weeks, is instantly added from a bottle. Again this is neither the right way nor the wrong way, only another option. However, it does come with a warning: it does seem to be associated with an increased incidence of dinoflagellates. The key here is that in order for the bacteria to proliferate fish must be present to create ammonia. When using bacteria from a bottle, fish have to be added at the same time.

How to set up a successful saltwater aquarium

If you are considering buying your first saltwater tank, or have just started keeping marine, download this ebook full of up to date information.

Or click here to find out more.


What is the fastest way to cycle a saltwater aquarium?

As explained above, using bacteria from a bottle is the fastest way to cycle a saltwater tank.

How do you know when your tank has cycled?

Your tank has cycled when you have established the nitrogen cycle in your system. When you have tested for ammonia and and nitrite and you cannot get a reading for them, and you have tested for nitrate and have started seeing a rise, the nitrogen cycle is complete.


The term die-off is associated with the transfer of established live rock from one aquarium or holding tank to another. While the live rock is out of the water bacteria and microorganisms will die.

It is widely advised that die-off is good, because in doing so you are introducing ammonia into a new tank. By introducing ammonia, you have done away with the need to introduce an external source of ammonia. The expectation is, and quite rightly, that the denitrifying bacteria will break down the ammonia, convert it into nitrite, then into nitrate.

As the denitrifying bacteria have also been introduced on the live rock itself, establishing the nitrogen cycle takes less time than the dry rock method took. Typically establishing the nitrogen cycle using live rock with a lot of die-off can take 4 – 6 weeks to cycle your saltwater tank.

cycle a saltwater tank
Dry rock added with ATM Colony

If die-off can be avoided, it should be

Typically, if you can get your newly acquired live rock into your tank within half an hour, you have successfully transferred far more living bacteria too. In doing so, you may get the smallest of ammonia spikes, followed by an equally small nitrite spike. Nitrate will become detectable within the second week … which means you can start adding your fish.

*Not all live rock has been removed directly from the sea.

Live rock is still readily available, but may not have been anywhere near the ocean. It could be dry aragonite which has been placed in a local holding container, allowing the bacteria and life in that holding container to inhabit it.

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