Cycling your marine tank

Taking the mayhem out of marine fish keeping


Cycling your marine tank


Although I have mentioned the cycling process in some detail on the ‘filtering your tank’ page it is such an important part of starting any marine tank I figured it justified its own page.


Although everyone will tell you that you can’t rush anything in this wonderful marine fishkeeping hobby of ours the cycling process is something that will take its own time, be ready when the process has finished and if you do rush it you will be subjected to all-sorts of horrors during the early stages of setting up your marine tank.

Cycling is effectively the process of establishing all the different bacteria that are necessary for the complete biological filtration of your stocked marine tank. You must always remember that these bacteria populations need time to adjust and grow to meet differing waste conditions so if your tank has cycled this does not mean there are suddenly enough bacteria to meet the waste produced by a fully stocked marine tank. It means that ‘starter’ populations have become established in high enough numbers to deal with the waste that is already in your empty tank.




Once these starter populations are established you can consider adding some livestock such as a small fish or some clean up crew, you will then need to wait a while to allow the bacterial populations to increase and adjust to the increased bioload before you consider adding anything else.


Cycling your marine tank


The process of developing these bacteria is called the nitrogen cycle. This is a chain reaction which results in the establishment of all the different bacteria required to carry out successful biological filtration.


There are three elements to the nitrogen cycle, the first, Ammonia, comes from waste in the aquarium. Ammonia at any level in the aquarium is highly toxic to your inhabitants and should only ever be present in the very early stages of setting the tank up. Sometimes people will get ammonia readings in an established tank and this usually means trouble. Either a fish has died and started decomposing before your clean-up crew have got to it or a complete failure of the biological filtration bacteria due to something like an unsuitable medicine treatment.


Once ammonia is present in the aquarium the bacteria that turn it into the less toxic nitrite will start to become established. These bacteria, called nitrosomonas, will start to establish populations able to turn the present quantities of ammonia into nitrite.


Nitrite is also toxic at low levels in your marine aquarium and should be regarded in a similar fashion to ammonia, again it should only be present during the early stages of setting your tank up and if it is recorded in an established tank then the cause should be found and rectified quickly. Nitrite is converted to nitrate by bacteria called nitrobacters. These bacteria require food (ammonia) and oxygen to survive. They will colonise every available surface of your marine tank and, once established in sufficient quantities your nitrite levels will start to fall and nitrate levels should start to increase. Nitrate can be tolerated at higher levels within the aquarium but most hobbyists try to aim for lower levels (between 1 and 3 ppm). Increased nitrates are often associated with increased phosphates and both, either individually or together, can cause unsightly algae growth.


Starting your cycle


Starting the nitrogen cycle in the first place requires ammonia, in the not too distant past people actually used fish to produce the waste (ammonia) and kick off the cycle. This is not considered good practise as any animal you use will be subjected to potentially fatal levels of ammonia and/or nitrite which at the very least is cruel. There are so many products on the market now that negate the need to use fish that there really is no excuse to subject any animal to that level of cruelty.


Under normal circumstances you will not even need to buy any of the above mentioned products. Assuming you are running a simple, standard tank (which is what this website is about) you will be using live rock as your main source of filtration. Live rock is not actually living but so called as it is the medium on which most of your bacteria will thrive. It’s made up of long dead coral skeletons (the type that make the coral reefs in the first place) and as such is very porous. Indeed it's weight is a good test of the quality of the rock.


Usually when you buy live rock, no matter how long it takes for you to get the rock in your tank there will usually be some die off. This is when the rock has been exposed to the air for a period of time and the bacteria and any sponges that had started to grow have started to die. Sponges will start to die as soon as they are exposed to the air so no matter what period of time the rock has been exposed you will always get some die off. The amount of die off can, of course, be reduced, you can transport your rock in buckets of salt water or even wrpped in newpaper. Anything that keeps the rock wet or damp will help to reduce die off.


Some die off, when starting a new tank, isn't necessarily a bad thing. You need some ammonia to start off your nitrogen cycle and its this die off that will give you that ammonia. After you have put your live rock in your new tank and got the pumps going your cycle will start immediately. After a day or two you should be thingking about starting to measure your levels. At the very start it won't hurt to test everything, this will give you a base line, from which you can compare all future test results.


Ammonia should appear first but depending on the amount of die off from your rock you may get low readings of ammonia and some nitrite. Assuming a normal cycle you should expect to see ammonia for the first couple of weeks and aas these levels start to drop nitrite will become apparent. After another couple of weeks nitrite should be dropping and you should start with the onslaught of nitrate. A typical tank has usually gone through its cycle in between four to eight weeks but every tank, every rock and every fish keeper is different. Regardless of what happens through your tank cycle the only constant is patience. Make sure your tank is fully cycled before you start to do anything.


Once your levels have sorted themselves, ammonia has been undetectable for a while, with no nitrite readings and nitrate of below 20ish you can test your tank bacteria. I throw some frozen fish food in such as a few mysis or brine shrimp and monitor ammonia for a few days afterwards. If ammonia remains at 0 then you are ready to start considering your first tank inhabitants.

Helpful tips




The shorter the time your live rock is exposed to air the shorter your cycle might be.


No matter how long the liverock has been out of the water some organisms will start to die immediately, such as sponges.

Cycling - in a nutshell

Never try and rush the cycling process, it will come back and bite you in the backside in the future.

The bacteria required to achieve efficient biological filtration will take time to become established, just because your tank is cycled doesn't mean your tank is suddenly ready for you to run out and fulfil your final stocking dream.

Cycling you tank takes three main componants. Ammonia, which is converted to nitrite which is converted to nitrate. All of these processes are carreid out by different bacteria.

Live rock is one of the most effective mediums you can use to help the development of the bacterial populations.

The die off from transporting your liverock to your new marine tank is sually sufficient to kick start your cycle.


You should start monitoring your water as soon as the cycle starts. This will give you a clear idea of the development of the bacterial populations.



You should be testing for ammonia, nitrite and nitrate during the early part of your cycle. Once ammonia and nitrite has dropped to 0 and you have nitrate readings your cycle is comming to teh end.

It won't hurt to think about checking for phosphates during the cycling process. If the live rock has been subjected to high levels of phosphates in the past it can absorp then and ralease them later on. If you detect phophates during your cycle you should think about rnning a phosphate removal media straight away otherwise you could end up with big algae problems down the line.