Filtering your marine tank

Taking the mayhem out of marine fish keeping

The Marine Tank Environment

Filtering your marine tank


There are thousands of pages all over the internet that will explain how to filter you marine aquarium an they will probably give you a lot more detail than I will here. The objective of this page is not to list every possible option but to simply explain a few details about a popular, tried and tested method of filtration that will suit most purposes.


The system I use along with most other marine keepers is called the Berlin method which relys on a very natural method of biological filtration using live rock and often a protien skimmer.


Live rock is peices of old coral skeletons (the type that make up the coral reefs) and looks like rock but is very light weight due to it being porous. It is its porosity that makes it so valuable as a biological filtration media.


A protien skimmer is a piece of equipment that helps to 'mop up' the rest of the waste in your marine tank that can lead to undesirable levels of nitrate and phosphate

Cycling your tank in the first place


Cycling is the name given to the process your marine tank has to go through before it is ready to accommodate any of your new inhabitants. Its full name is the nitrogen cycle that turns ammonia (a product of waste from the marine tank inhabitants) to nitrite to nitrate which is then converted to nitrogen gas and further diluted in the marine tank environment by regular partial water changes. There are other, more involved ways to get rid of nitrate in the marine tank but I'll detail these on another page.


The purpose of live rock in the marine aquarium


The nitrogen cycle is carried out by a suit of bacteria which develop and build up on all the surfaces inside your marine tank. Different bacteria carry out different parts of the cycle and have different requirements. Some will live deep within the live rock, some on the outer surfaces, the benefits of using high quality live rock is that it is very porous therefore offers ideal conditions for all these bacteria to thrive by offering a high surface area. One of the bacteria types (nitrsomonas) turns waste in the form of ammonia into the slightly less toxic nitrite. Once nitrite levels are detectable a different bacteria (nitrobacters) will start to establish themselves. its the nitrobacter bacteria that produce the waste you will read as nitrate readings in your marine tank. Once these nitrate readings are starting to climb and ammonia and nitrite reaadings remain at 0 you know the bacteria populations are increasing and doing their thing. You must remember, however, that these populations are still only small and are no where near the levels that will be achieved over time in a fully stocked aquarium so dont be tempted to charge out and buy a shed load of animals for the tank otherwise your cycle will start all over again.


Starting the cycle


Once you are actually at the stage of filling your tank with salt water made up from good quality RO water and a decent branded marine salt you can think of adding your live rock. Where you purchase you rock from is entirely up to you but it is worth keeping an eye out on local classifieds as it often comes up for sale at around £5-8 per kilo. Fish shops often sell it for between£10 - 13 per kilo. It is worth pointing out, however, if you buy it privately there may be some uncertainty of its history, has it ever been in a tank treated with copper? Did its original tank suffer from high levels of Nitrate or Phosphate? (live rock can leach phosphate out for months, thus promoting an excessive growth of undesirable algae). Has it ever been exposed to any diseases? The good points of buying it privately are that it may already come with all sorts of hidden goodies such as little coral frags still attached. So rock from a shop will probably cost more but you will have more confidence it is fit for purpose and you may have more choice of interesting shapes and sizes from which you can build a more interesting aquascape.


Where ever you have purchased your live rock from as soon as it goes in your tank the cycling process will begin. Live rock is so called because of the live bacteria populations both in it and on it. As soon as this rock is taken out of the water these bacteria will start to die. This is referred to commonly as 'die off' and the extent of this die off will influence your tank cycle. Die off can be reduced by transporting your rock in buckets of original tank water (not always practical) or at least wrapping it in newpaper or plastic bags to reduce its contact with air.


If we assume an average scenario of getting your live rock from somewhere down the road and it takes an hour to get it from its original tank to yours then you will get some die off. Once you have put the rock in your tank of fresh salt water it will be this die off that will produce ammonia and thus start your tank cycle. Years ago people used a live fish to kick start the cycle but this seen as cruel as the fish will be subjected to high levels of very toxic ammonia that will ultimately burn its eyes, gills and just about everything else, it is also completely un-necessary.


Chances are your live rock will have some of the necessary bacteria still surviving on it so once you have put the rock in the tank it is a question of patience (and sometimes lots of it). There are a lot of products appearing on the market now that claim instant cycles or at least to be able to speed them up, I haven't used them so not in the position to comment but as this website is about simple and cheap I'll leave the details of those products to someone else, suffice to say they do exist.


Whilst you are patiently waiting for your bacteria to become established it's time to get familiar with your test kits. For the cycling process you need to be able to accurately test for Ammonia, Nitrite, Nitrate and ultimately phosphate. If you are keeping a simple tank like FOWLR or soft corals then chances are these are the only test kits you will ever need.


You shoud be prepared to test regularly and frequently during the cycling process to get a real handle on whats going on. I would suggest at the very beginning to test for Ammonia, Nitrite and Nitrate for the first few days then concentrate on ammonia until this is getting to 0, then move on to nitrite and as the nitrite levels start dropping start testing for nitrate. You should also keep an eye on phosphate levels as there is a chance the live rock could start to leach phosphate into your system. At the first sign of phosphate you should consider running some sort of phosphate removal media (see the equipment list page). If you don't deal with phosphate during the early stages of setting your new marine tank up it will come back and haunt you at a later stage (trust me it will....)


I'll mention protien skimmers here for completeness but see the equipment list page for more detail. A protien skimmer is a cracking piece of kit and you should consider using one in the early stages of your tanks journey. Although much of your tanks filtration will be carried out by the processes mentioned above the protien skimmer will help to keep nutrient levels that bit lower by removing additional waste from the water. if you run a sumpless tank like I do a protien skimmer can also provide a handy place for any chemical media (should you decide to run any) and filter floss (great for giving your tank water that crystal clear look)


Helpful tips


Filtering your marine tank


Always try and get the highest quality, lightweight live rock you can, this will always perfom better as a filter than heavier, denser rock


Don't be tempted to buy ocean rock for your tank, you could probably get away with a couple of pieces to bulk your aquascape out but remember that it will serve no filtration purpose and will be taking up space that could be better used for additional filtering rock



Filtering your Marine Tank - in a nutshell

One of the most commonly used methods of filtering your tank is called the Berlin method which relys on bacterial populations in and on live rock and is often complimented by the use of a protien skimmer

Always make sure your tank is fully cycled before thinking about introducing livestock, you can check this by putting something like a small piece of prawn into the tank and measuring the nutrient levels, if ammonia reappears the bacterial populations haven't become established enough to support livestock yet

Although I will provide more details on the 'stocking your marine tankk page' just because your tank hasa cycled doesn't mean you can start stocking in large numbers straight away

If you buy live rock privately be sure to ask questions about it's history, has it ever been exposed to copper or disease, has it been sitting in a tank with high nitrate levels?

Any die off on your live rock should be enough to kick start the cycling process, the less die off, potentially the smaller the cycle.


The length of your tank cycle will vary, no two tanks are the same it could take a fortnight, it could take six weeks, it all depends on the quality of the rock and the amount of die off it experienced during transit.

Don't be alarmed if your tank starts looking like an eyesore during and just after the cycling process, you can expect al sorts of undesirable things to start appearing such as cyanobacteria and diatoms, not forgetting our old friend the hair algae. Don't panic, all these things are normal in the early stages and as your tank starts to stablise they should start to receed and give way to the thing of beauty you always imagined

Don't ignore phosphates, as soon as they are recorded you should add some sort of removal media

After the rock, powerheads and of course the tank itself, a protien skimmer is reallly the most essential piece of kit you will own of you are keeping it simple