How to Cycle Your Saltwater Aquarium With Live Rock

Prepare Your Saltwater Aquarium for Use

Live rock aquarium
Moto "Club4AG" Miwa/Flickr/CC BY 2.0

The most important factor in keeping aquarium fish healthy is good water quality. In order to do that good bacteria must be established in the aquarium. Beneficial heterotrophic bacteria living in the aquarium gravel break down fish wastes and other debris, and nitrifying bacteria, living in the filter media and on other surfaces, convert toxic ammonia produced by the fish into nitrite and then to nontoxic nitrate. These beneficial bacteria take some time to establish themselves in an aquarium. New aquariums must go through a cycling process before any fish can be safely added to the environment. This process leads to the growth of the bacteria in the aquarium's "biological filter." From start to finish, this cycle usually takes around 30 to 45 days to complete, and depending on each individual aquarium's set up and care variables, sometimes longer.

If you don't want to wait around for nature to run its course and create this needed bacteria, there are ways to help speed up the cycling process by "seeding" a new tank with media or materials that already have a mature beneficial bacteria population established on them. Seeded live rock is an excellent source of nitrifying bacteria and due to its porosity, it can carry a huge quantity of bacteria that will be buried in all of the nooks, crannies, and minute holes in the rock.

A biological bacteria seeding method will help to cut a substantial amount of time (weeks, in most cases) required to establish the biological filter in a saltwater aquarium, which is considered to be "cycled" when the different species of bacteria (Nitrosomonas, Nitrobacter and Nitrospira) are present in quantities that will process the ammonia (NH3) into nitrite (NO2-) and then into nitrate (NO3-), leaving no detectable ammonia or nitrite. 

Hermit crab
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Before You Begin

It may take a saltwater tank longer to cycle compared to a freshwater tank. Keep in mind you'll want to allow for at least six weeks for your tank to cycle before purchasing all the fish you will want. You must add the fish only a few at a time into the aquarium during the cycling process to not overwhelm the growing nitrifying bacteria. It's important to monitor and test your water often throughout the cycling process so you can properly time when you're ready to add more fish. Do not add fish if the ammonia or nitrite levels are high.

What You Need

Gather the following items when you're ready to cycle your saltwater tank:

  • A new saltwater tank fully set up with all desired contents except for the fish. This includes your filtration system, heater, thermometer, decorations, plants, and other accessories.
  • A seeding source such as live rock or media from another saltwater tank. This will provide the beneficial bacteria faster than letting it naturally grow in a new aquarium.
  • Your other option is to use a bottle of pure ammonia. If using ammonia, make sure that it does not include perfumes or surfactants (detergents). This option can be used to stimulate the nitrifying bacteria to grow using ammonia as a food source before adding any fish or invertebrates .
  • An aquarium water testing kit to monitor the amounts of ammonia, nitrite, and nitrate in the water.

Locate the Seed Sources

Seed sources are usually obtained by removing them from another saltwater aquarium that is well established. Ideally, the seed source should be at least six months old. It should also be disease free, meaning that it is not suspected of having a disease or is undergoing treatment for a disease of any kind. Good sources for seeded live rock are a local fish store, if rock that has been in a functioning saltwater aquarium is available, or a friend's tank that has been running for a period of time. Many fish stores sell cultured live rock or coral sand as a way to seed new aquariums.

There are also commercially prepared live bacteria products available at fish stores that can be added to a new aquarium to get the beneficial bacteria growing. These are not as good as adding live rock to the aquarium, but do help to reduce the time it takes for a new aquarium to cycle.

Transfer Live Rock From One Tank to Another

When transferring the live rock from the established tank, many people will scrub the rock to remove any algae and dead or decaying material. While removing as many unwanted "hitchhikers" (such as bristleworms or mantis shrimp) as possible is a good idea before transferring the rock, it is usually best to not scrub the rock to remove all of the algae as this will remove a lot of the beneficial bacteria, which will defeat the purpose of using the seeded rock. Be sure the live rock stays wet or the beneficial bacteria can die, and do not use chlorinated water to rinse the rocks.

Other tank materials such as live sand or coral sand substrate or even aquarium decorations from another established saltwater aquarium can also be used to transfer live bacteria and establish the biological filter bed in a new tank. These materials will carry the same bacteria as the live rock and will help cut the cycling time of the new tank. 

Nurturing Live Rock in Your Tank

Once the live rock is transferred to the new tank, the bacteria will require a food source (ammonia) in order to reproduce and populate the surface areas in the tank. While it is safe to place a few fish in the tank to provide the ammonia source, it is best not to load up the system with livestock too quickly as this will produce more ammonia than the relatively small population of bacteria can immediately handle. only add a few fish at a time over several weeks while monitoring the water quality. When the ammonia and nitrite levels are normal, you can safely add a few more fish.

There are a number of other sources for the ammonia required to feed the bacteria, other than just fish. Invertebrates such as hermit crabs, snails, and shrimp that you are planning to add to the aquarium will consume food and produce ammonia. Just placing these Reef-Tank-Safe Janitors in the tank ​will go a long way toward reducing tank maintenance in the future.

Using Ammonia to Cycle Your Saltwater Tank

If you don't want to use media from another tank or don't have access to a natural seeding source, you can also start the cycling process by adding drops of pure ammonia to your tank. To make sure there are no detergents in your ammonia, shake the bottle before using. If bubbles start to foam after shaking, it means detergents are present and you'll want to find an alternative ammonia source that's pure. Once you have secured an unadulterated bottle, add five drops a day for every 10 gallons of water to your empty tank to start the cycling process. When your test kit starts to detect nitrite, cut back to only three drops a day. Do NOT add any fish or invertebrates to the aquarium if you are using ammonia to start your bacteria cycling.

Once the tank has reached its peak readiness, meaning your water test reads zero ammonia and nitrite, and nitrate is above 1 ppm (mg/L), you'll want to stop adding ammonia. After you stop adding ammonia, wait a day and retest your water before you add your fish. Both ammonia and nitrite can kill fish, so you want to be sure they are completely removed from the water before you add the first few fish to that tank.

Preventing Problems With Your Tank During the Cycling Process

Testing the aquarium water for ammonia, nitrite, and nitrate on an almost daily basis will help you to determine how well the bacteria population is increasing in a new aquarium. If the ammonia level is rising and approaching the danger zone, you can quickly reduce the level with a dose of an ammonia neutralizing product such as Amquel to keep the level in the safe zone.

The goal is for the bacteria to quickly break down all of the ammonia to nitrite and then to nitrate, so that the water tests are nearly zero for ammonia and nitrite. The nitrate will gradually accumulate in the aquarium water, so water changes need to be made to remove the nitrate. Keeping the nitrate level below 20 ppm (mg/L) is healthiest for the fish and invertebrates.

If you suspect your pet is sick, call your vet immediately. For health-related questions, always consult your veterinarian, as they have examined your pet, know the pet's health history, and can make the best recommendations for your pet.
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  1. Aquarium Water Quality: The Nitrogen Cycle. Florida Department of Agriculture and Consumer Servcies.