Cycling a Marine Tank

By Karen Barber

What is cycling?

Even the most sophisticated equipment and additives available these days can't take the place of tiny bacteria in keeping our tanks clean and animals healthy. 'Cycling' is the process of introducing and establishing these essential bacteria.

In a nutshell, all fish and invertebrates, including seahorses, excrete ammonia as a waste product. However, the ammonia is extremely toxic to them. Fortunately, there is a group of beneficial bacteria that can convert ammonia into nitrites, but nitrites are also toxic to our livestock. The good news is there is a second group of microbes that can convert nitrites into nitrates, which are not nearly as toxic. Accumulated nitrates should ideally be kept below 10 ppm, but fish and seahorses may show no ill-effects with slightly higher levels.

To cycle a tank, these two groups of bacteria need to be present and established in sufficient numbers prior to adding your seahorses or other animals. Ammonia levels will rise, followed by a rise in nitrite levels as ammonia begins to decline. Then, as nitrite levels begin to decline, you will more than likely see a very slow and gradual rise in nitrates. The appearance of nitrates is a good sign that your various bacteria are making all the necessary conversions, however, some tanks with a significant amount of live rock, live sand, macroalgae, and/or are regularly maintained with water changes may never see nitrates accumulate.

How do I do it?

There are several methods people employ to cycle a tank, however, this article will primarily focus on two of them — live rock and the ammonia method.

Prior to getting cycling underway, make sure your tank is up and running and your equipment is properly functioning. Circulation, aeration, and a stable temperature are essential factors for the health of your livestock long-term, as well as at the very beginning when you are establishing your bacteria colonies. It is also a good idea to turn your tank lighting on for several hours each day during cycling, just like you will when the tank is stocked.

Cycling with live rock

One of the best ways to cycle a tank is through the introduction of live rock. These are natural pieces found in the ocean and contain a multitude of life in and on them to get your cycling underway. Placing live rock into your new system jump starts the cycling process. Ammonia is produced as some animals on the rock die off and others excrete waste. In addition, the live rock also contains the beneficial bacteria groups needed to convert ammonia to nitrites, and nitrites to nitrates.

The time it takes to cycle a tank with live rock varies on how 'fresh' the rock is. Rock recently harvested from the ocean (not 'cured') will have a great deal of life on it that will not survive the rigors of transportation and being removed from the ocean. This rock will have a lot of die-off that will trigger significant ammonia and nitrite spikes which can make the cycling process last a few weeks.

There is also pre-cured live rock, which means that the vast majority of the die-off has already occurred before you receive it, however, further transportation can still cause smaller ammonia and nitrite spikes. Cycling with pre-cured live rock can still take 2–4 weeks or so.

Then, finally, there is cured live rock. Fully cured rock that is quickly placed into your tank with minimal transportation time, such as from your LFS (local fish store) to your home, may 'instantly' cycle your tank. The beneficial bacteria groups are already there in good numbers with no die-off to trigger ammonia spikes of any significance. With fully cured live rock you may never see the typical cycling progression of ammonia —> nitrite —> nitrate. Your levels may initially read — and remain at — zero, even while you are slowly stocking your tank.

To cycle your tank with live rock, an effective amount to start with is 1 pound or more of rock per 5 gallons of tank volume. If you plan on utilizing live rock as your chief method of filtration, approximately 1 pound of rock per gallon of water is ultimately recommended.

Please note that you should NEVER add uncured live rock to a stocked, established tank. Either cure the rock in a separate container, or make sure to purchase FULLY CURED live rock with a very minimal transportation time from your LFS to your tank.

After adding live rock to your tank for cycling, test for ammonia and nitrite daily or every other day with test kits available at your LFS. With rock not fully cured, you will see the typical cycling progression. Then, once ammonia and nitrites test at zero for several days in a row, the tank is considered cycled. The time frame for this to occur can vary widely. Factors such as the quantity and condition of the rock, as well as overall tank health and stability (circulation, temperature, aeration, etc.) will directly influence the time it takes for the bacteria to sufficiently reproduce and colonize. Testing is the only way you will know what is going on with your tank.

Cycling with the ammonia method

Another method of cycling a tank that is increasing in popularity, is using 100% pure household ammonia. Instead of utilizing ammonia-producing organisms to start the cycling process, the ammonia method involves directly adding bottled ammonia along with a starter culture of bacteria.
The ammonia must be clear, unscented, 100% pure, and with no additives. However, you may find typical household ammonia to be mostly water with an actual strength of only 5–10%. Since the concentration may vary, you will need to experiment a little to find the right amount needed to cycle your tank.

When using the ammonia method, the necessary bacteria can be introduced to your system a couple of different ways. If you have access to an established and healthy tank, obtain a small amount of sand or gravel or even used filter media. This can then be placed in the tank to be cycled and will provide a starter culture of bacteria. Another option is to use a commercial preparation of bacteria, such as Hagen's Cycle, to introduce the nitrifying microbes.

After the tank has been set up and the bacteria have been introduced, begin to add the ammonia. If the tank is on the smaller side, start with only a couple of drops. Let it circulate in the water for a short time, and then test for ammonia. You want to raise the ammonia level to about 1–2 PPM Continue adding drops of ammonia and testing, while keeping track of how many drops total it takes to get to 1–2 PPM

Once you've reached the desired level, you now need to wait for the bacteria to multiply and do their job. It may take several days to a couple of weeks for your ammonia test to register zero. Once it does, add the same number of drops of ammonia again. Keep repeating this many times, each time waiting until ammonia reads zero, until finally the ammonia test goes to zero within 8–12 hours of the addition of ammonia.

When the tank is able to clear the ammonia within this time frame, that means that there are large numbers of bacteria present. At this time, test for nitrites. If nitrites are present, perform 3v4 more ammonia cycles to further establish the second group of bacteria. If the nitrite test reads zero, perform a very large water change and then you can begin slowly stocking your tank.

It's been suggested that elevating the temperature of the tank to the mid-80s F during cycling with the ammonia method can have beneficial effects. Just make sure that there are no animals in the tank being subjected to this temperature extreme (and ammonia), and also make sure to lower the temperature again prior to stocking.

Also, tanks with low alkalinity may experience pH spikes with the addition of household ammonia. These spikes may also hinder the growth of the necessary bacteria. To counteract the problem, use a good buffering product prior to cycling with ammonia, and perform a large water change after cycling (prior to stocking) to get pH levels back on track.

What about cycling a tank with fish or other animals?

Several decades ago, when people were first setting up marine tanks, the conventional method of cycling was to use a hardy fish or other animal to introduce ammonia into a new system. However, by using live animals you may be introducing a parasite or pest to your new tank. In addition, the animals are being subjected to toxic ammonia and nitrite levels, which is unnecessarily cruel. When there are other safer and gentler ways to cycle a tank, there is no need to use live animals.

When can I start stocking?

Once your ammonia and nitrite levels are at zero, whether it be from cycling with live rock or several cycles with the ammonia method, you are close to being able to stock your tank.

If you used a skimmer during cycling with live rock and/or did not have extreme ammonia or nitrite spikes, you will probably only need to perform a standard 10% water change prior to stocking. If you did not use a skimmer and/or you registered significant ammonia and nitrite levels with a lengthy curing/cycling process, you will need to perform a more significant water change, such as 50–75%.

As mentioned earlier, since the ammonia method may cause problems with pH, a very large water change, approximately 90%, will be necessary following the use of this method, prior to the introduction of animals.

When ammonia and nitrites test at zero, nitrates are <10 PPM, pH is between 8.0–8.4, water changes have been performed, and equipment is functioning properly, you are ready to start stocking your tank.

Try to keep nitrates at 10 PPM or less. If nitrates are high or continue to climb over time, more frequent water changes and/or the addition of macroalgae will help bring them down. If your pH is below 8.0, add a high quality buffer to raise your alkalinity and pH.

How do I stock?

Patience is a virtue with marine tanks, and stocking too many animals too soon can have disastrous effects. Start with a very small number of animals (acclimated properly), and test the water daily for several days after introduction for any ammonia or nitrites. If ammonia and nitrite levels consistently read zero, continue to to wait for at least two weeks before adding any new animals to allow time for the bacteria to multiply and effectively handle the new load.

If you ever find your ammonia and/or nitrite levels approaching .4 PPM or greater in a stocked tank, you will need to act fast. Diluting the toxins in the form of one or more large water changes, adding more bacteria (live rock, 'seeded' media, commercial products, etc.), the addition of macroalgae or a chemical filter to absorb the toxins, or temporarily relocating the animals to another tank are all effective ways to handle an ammonia or nitrite spike.

Do I need to test after cycling is complete?

Even after cycling is long over, it's a good practice to periodically test for the presence of ammonia, nitrite and nitrate, especially after adding new livestock. Equally important is knowing your tank's pH range, and making sure that it stays between 8.0–8.4. (Your pH will typically be lower in the morning before your lights come on, and higher in the evening just after the lights go off. )

Catching a potentially serious problem early and taking immediate steps to correct it can help prevent a lot of heartache.


Most recent revision: 2003

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Karen Barber &
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