Cycling a Marine Aquarium

I just thought I would take some time to explain some of the methods that can be used to cycle a marine aquarium in a little more detail that what we currently have on our forum. The nitrogen cycle in a marine environment is similar to that of the nitrogen cycle in a fresh water environment. In both environments you have nitrifying bacteria that coverts ammonia into nitrites and nitrites into nitrates. While the nitrifying bacteria in a marine aquarium are different bacteria then the nitrifying bacteria in a fresh water tank, they do perform the same functions and essentially work the same. A marine nitrogen cycle has the addition of de nitrifying bacteria which converts nitrates into nitrogen gas. I will be making many comparisons between the fresh water and salt water cycling methods through this article in order to better illustrate some points better for people who are new to salt water aquariums.

I do not suggest cycling a marine tank with fish for two reasons. The first reason is that there a very few marine fish that can tolerate lower levels of ammonia in the water. The second reason is the larger water changes required to keep the safe for even the hardiest of fish would make it very difficult to maintain your PH stable enough for any marine fish not to mention the extra cost of the salt.

The first step is to decide what type of biological filter media you are going to use. Regardless of what type of biological filter media you choose, you will need to cycle your tank before adding any type of stock to the tank. The two most common methods used are: biological filter media just as you would use in a fresh water filter, and rock (both live and dry) from the ocean. Each has their good and bad points. For any cycling method you want to use a good quality salt. While cycling (regardless of the exact method), you will need to maintain your salinity between 1.025 and 1.027 and your temperature between 78F and 82F. You won’t have to worry about testing for alkalinity, calcium, or magnesium until after the first part of your cycle is finished.

Cycling with Biological Filter Media

This process is very similar to the freshwater fishless cycling process we have on our forum which is described in the below link:

This process is best suited for when you want to use a biological filter media in a typical hang-on-back filter (HOB) or a canister filter just as you would expect to use on a fresh water tank.

You will need to add a source of ammonia to start growing your beneficial bacteria (BB). You can get this from: adding a piece of raw shrimp, adding fish food, or by adding pure ammonia. It is best to use pure ammonia so you can have better control over the process. When using pure ammonia, start the process by bringing your ammonia up to around 2ppm. A little higher is OK as well. As every manufacture makes there pure ammonia at slightly different concentrations, there is no set amount per gallon to add. It is always best just to add a very small amount, wait an hour or so, test your water, repeat as required. Once you have worked through that initial dosing, you will now have an accurate measurement of how much ammonia to add to bring your level up to 2 ppm. Check your ammonia levels daily. Once you start to see your ammonia drop, dose once each day to bring it back up to around 2 ppm. At this point you start testing daily for nitrites as well.

When your nitrite level spikes around or over 2ppm, start maintaining your ammonia level around 1ppm. Only dose ammonia once each day to bring it back up to 1 ppm. At times your ammonia will fall to 0ppm and that is OK provided that you bring it back up to 1ppm only once each 24 hour period. When you start to see you nitrites falling, start testing for nitrates as well. When your ammonia and nitrites are both 0ppm within 24 hours after dosing your ammonia back up to 1 ppm, and your nitrates are rising, you have completed the first part of a typical cycle and now have successfully grown the nitrifying bacteria you need. You can now slowly start stocking your tank. If it will be a while before you will be adding any fish to the tank, you will need to add a drop or two of ammonia to the tank to keep the bacteria feed and alive.

The benefits of this approach are that it is easy to do and it is the lowest cost method to cycle a tank. The drawbacks of using this method is that your filter media will require frequent cleanings to avoid any build up of crud in the filter media which is extra work and will limit any growth of the denitrifying bacteria. Using a filter can also be problematic for reef tanks making this approach more suited for fish only set-ups.

Cycling with Dry Rock

You should first cure your dry rock by soaking it in dechlorinated fresh water until all the nitrates and phosphates have been leached out. The below link explains that process

Cycling with dry rock is basically the same as cycling biological filter media. Your dry rock is the biological filter media in this case. As a general rule of thumb, you will need ~1 lb of rock for each gallon of water. Follow the exact same process as described for cycling with biological filter media. Once you have completed the first part of your cycle and added your fish, keep checking your nitrate levels. Within a few weeks you will notice a drop in the nitrate levels as the denitrifying bacteria grows. I have personally used this approach and it works well.

The benefits of this approach is that over time, the denitrifying bacteria will grow on your rocks and help you control your nitrates. There also is very little maintenance involved with using rock in your tank. You can also use this approach for reef and fish only set-ups. The drawbacks can include the extra cost as compared to using a filter and a little extra time curing your dry rock before adding it to the tank.

Cycling with Live Rock

This is my preferred approach to cycling a tank. When compared with cycling a fresh water tank, cycling with live rock is like adding both a source of ammonia and using already seeded/cycled filter media. When cycling with live rock, you should still use the general rule of thumb of having ~1 lb of rock for each gallon of water in your tank. The information on curing live rock (link below) explains the rest of the process.

When it comes to live rock, the process of curing the rock is the same as cycling the rock. The only difference here is that you do not want to add any ammonia while the tank is cycling. You still have to check your ammonia levels daily to make sure your ammonia will not get above 2ppm, and completing a water change if it does. You also have some options to keep your costs down. You can use partial amounts of live rock and dry rock to cycle your tank provided you use a minimum of 20% live rock. Once the first part of the cycle is complete, you’re ready to add fish. If the tank is going to sit empty for a while after the first stage of the cycle is complete, you should add a drop or two of ammonia every day to keep the bacteria fed and alive, but it should be OK for a least a few weeks / months before you would have to worry about doing that.

Note: it is always best to cure your live rock outside of your set-up.  A quarantine tank without any lights would work best.  This will help you latter to control your nitrates as well as help you avoid introducing something bad into your set-up


The benefits of this approach are the shortest time to cycle a tank as well as the shortest time to grow your denitrifying bacteria. Sometimes the first part of the cycle can be as short as s few days. You can also use this approach for fish only or reef tanks and is the best suited one for a reef tank. The only potential drawback is that live rock can be the most expensive approach to obtain biological filter media.


Please feel free to start a thread in the forum to ask any questions you may have.  If you are not already a member, you will need to sign-up first.


13 thoughts on “Cycling a Marine Aquarium”

  1. Marry Welson

    Thanks for the useful article about how to cycle a marine aquarium! I will need this article in some more day to cycle my aquarium.


    1. Last

      First off, I would like to suggest, The coseoinnticus Marine Aquarist’ by Robert Fenner. This will have everything you need to get started. Clownfish are a great beginner fish because they are fairly docile and easy to care for. Anemones are not required for the clownfish to thrive. Please note that anemones require pristine/stable water conditions and strong lighting (metal halides). I would not get one until you are more experienced with running a saltwater tank. Blue linkia starfish on the other hand are extremely difficult as they require a really slow acclimation to a very well established tank (2-3 years old). Even then, many wind up slowly starving to death in a home aquarium. It’s best to leave these guys in the ocean.You should only add invertebrates (Shrimps, snails, hermits, etc) only when the tank is fully established. Inverts are very sensitive to change in water chemistry and will react negatively to even trace amounts of ammonia and nitrite. Add these guys after your tank is fully cylcled and running for a couple of months. Patience is key in this hobby. I would suggest a mix of turbo snails, cerith snails, Hermit crabs, and a cleaner shrimp, fire shrimp or peppermint shrimp. The size of the tank will dictate the size of the tank you should ultimately buy. For example, it you wanted to have a pair of clowfish, a royal gramma, firefish and a pair of Anthias, you can get away with a 55gallon tank. If you want to raise Tangs or dwarf Angelfish you would definitely have to go 100 gallons or greater. Bigger is better in this hobby IF and only IF you perform all the regular maintenance. Aside from the tank, stand and salt. You need to decide what type of tank you intend on having, a fish only or a low light reef tank (softies; mushrooms, leathers etc) or a high light reef tank (Stony Corals, clams). If you intend on keeping any corals, I would suggest to get the best lighting you can afford. Power compacts or VHO’s for softies and Metal Halides for the higher light loving creatures. Aragonite sand and live rock are important components to your biological filtration and help buffer your water to ~8.2. You would also need to get yourself a skimmer to help pull out the dissolved organics from the water and some type of mechanical filtration (Canister filter). Don’t underestimate the usefulness of test kits. In this hobby they are probably the most important piece of equipment because they will alert you to harmful changes in the water chemistry. I would suggest 2 heaters that are rated for half your tank to provide insurance’ if one should fail. For example, if you have a 100 gallon tank, get 2 heaters rated for 50-75 gallons. And finally you would need to get yourself a refractometer and thermometer to measure specific gravity and temperature. Hope this helps. Best of luck


      1. Ivan

        Quick question, Steelo .I am tiyrng to battle old tank syndrome. If I replace 80% of my old rock with new live rock, can I take 20% of the old rock and let it sit outside for a week or so and then add back into my tank as base rock? Will this newly formed base rock still leach out phosphates? Thanks!


        1. Cliff Erhardt Post author

          Ivan: drying out you rock won’t help to release the nitrates and phosphates that have been soaked up into the rock. I would suggest recuring the rock and testing for nitrates. Once the nitrates test at 0ppm, it will be ready to put back into the tank.


  2. Daniel J

    EVERYONE has their own opinion of how long a tank need to wait for a full cycling to occur. As a Marine Biologist, let me assure you…what I am posting is a proven scientific fact.
    Waiting a minimum of 6 weeks for a salt tank to cycle, is an absolute proven MYTH. Using live rock, is not a good idea, as it introduces a number of bad organisms and parasite/pests to your new Aquarium. Using Damsels to help cycle your tank, is not always a good idea either, if they survive the ammonia and nitrite and nitrate spikes, they will be very aggressive to any other fish you introduce later. I suggest Chromis, or tank raised percula clowns (although also in the damsel family) they are much more gentle than their close relatives.
    A tank can actually fully cycle in less than two weeks, using ANY form of filtration, wet/dry, canister…bio wheel. Not one is any better than another. It is always a matter of preference.
    Add base rock, and introduce some fish food starting on day one, and feed empty tank every day for one week…very small amounts. This will introduce a bio decaying process that will allow ammonia to build. Mother nature will do the rest.


    1. William

      A tank CAN fully cycle in less than two weeks. That does however not mean that you can recommend cycling a tank for only two weeks. You need to have margins on your side. Especially when giving advice to others. We all know that experinced aquarium keepers do not allways follow the advice they give to other less experienced aquarium keepers. This is not because they are dishonest. It is because they can ignore the advice since they know what to do if something goes wrong.

      Cycling your tank for 6 weeks give you margins to work with if something goes wrong. Cycling for two weeks requires everything to work out perfectly or the tannk will still not have fully cycled after two weeks. the responsible thing to do is therefore to recommend that people, especially beginners, cycle for longer than might be necessary if everything work out as indeended.


  3. Cliff Post author

    Good comments William
    I always feel no matter how you cycle your tank,it is always best let your test results tell you when your cycle is done. Once cycled, take things very slow as your tank becomes mature and stable over time.


    1. Daniel J

      I agree 100%….one would hope that the usual impatience of a lesser experienced Aquarium owner will not occu,r and do what most do, and that is to start overloading the tank too early with a bio load that will become sick, diseased and start a legacy of an unhealthy tank that will never recoup. I have to say from experience, the purchase of live rock…or what most aquarium stored try to sell calling it “premium live rock” for $10.00 a pound and up…is NOT RECOMMENDED, it will 9 times out of 10 introduce things into your enviroment that will never allow your tank to have a chance for a really healthy start. Yes, most want the purple healthy looking algae right off the bat. EVERYTHING….even in spite of my initial post…everything in regard to your Aquarium requires patience and observation along with simple tests….but there are some that are firm in believing that water changes every week or two is a good idea…will it hurt?…no…but it will stress the bio load…and is a HUGE waste of time. If you do not over feed your fish, and have a protien skimmer for a ny size tank, 30gal and up….between this and your filtration, healthy water is re introduced into your tank constantly along with the proper gas transfers allowing oxygenation infusion back into the enviroment.


      1. Daniel J

        Oh, and my reference to mother nature…lol….was referring to the development of beneficial bacteria that naturally occurs creating a controlled Eco system within the tank. Other than that…mother nature has NOTHING to do with the tank.


  4. Aaron

    Thanks for all the good info. I am new to this hobby and just have one question. I just bought a 30 gallon tank with bio-balls as a filter media. I was thinking about getting live rock also. Would this be too much filtration and create too high of nitrates in the water? I don’t want to overdo it, is it possible to overdo it or am I OK with the bio-balls and live rock? Thanks


    1. Cliff Post author

      Personally, I would recommend using only live rock for your biological filteration. It will help to reduce your maintenance as well as helping your dKH and Cal levels stabilize. Just make sure you get enough good quality live rock (very pouris)


  5. tom szuminski

    why does the– API saltwater master test kit– nitrite levels only measure up to 5.0 ppm? I’m graphing my cycle and it is frustrating to me that I cant watch what is going on with it. From day two the nitrite has been a constant 5.0. I’ve been testing everyday for 8 days now. Everything else is showing progress but my nitrite levels


    1. Cliff Post author

      API test kits (IMO) were never really made to be highly accurate, just accurate enough to allow you to know about any significant changes in your water parameters. In my experience, the salt water API nitrite test is not very accurate. I think there is something else in a typical salt water tank that interferes with this test kit (just a guess on my part). However, as you can see the changes in your ammonia and in your nitrate, we will have what you need to verify what your cycle is doing. Salifert makes a very accurate nitrite test kit and is one of my go-to test kit brands


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