You have a few different options when it comes to providing proper filtration for your marine aquarium.  The below are the common approaches to filtration in marine aquariums that I have used as well as some pros and cons.

There are three categories of filtration that you need to look at, biological, chemical, and mechanical filtration.  Each has a slightly different purpose in your aquarium.

Biological filtration

The term biological filtration refers to the process of live bacteria converting organic waste in the form of ammonia and converting it into nitrites and further converting that into nitrates.  This is also referred to as the nitrogen cycle. The two most common approaches to biological filtration are to use either  live rock , or a filter with biological filter media (like you would find in a fresh water aquarium).

  1. Live Rock: In the ocean, the rock formed from the calcified skeletons of corals provides a suitable media for the beneficial bacteria to grow on, which is what actually processes ammonia and nitrites and is a more natural way of providing biological filtration.  There are many other benefits of using live rock in a marine aquarium that are discussed in the below link.  This is always my preferred method for biological filtration and I always recommend it.
     
  2. Filter(s) with biological filter media, or biological filter media in a sump can be very common to find in use in a marine aquarium.  This is my least preferred method.  While it certainly provides a lot of surface areas for bacteria to grow and can be very  effective at converting ammonia and nitrites, the filter media will require additional maintenance in order to keep the media clean and to help prevent the media from adding higher than normal amounts of nitrates in the water.  In addition, you will not be able to grow large enough colonies of the nitrate eating bacteria (discussed a little latter in this article).  Many hobbyists do take this approach with no long term negative effects, just some extra weekly maintenance.

Chemical filtration

Chemical filtration in a marine aquarium refers to using a filtering media which absorbs certain unwanted elements or certain pollutants out of the water.  One of the most commonly used chemical filter media is the use of carbon to remove toxins from the water when/if some of your corals release toxins as a defense mechanisms into the water which can harm the rest of the marine life in your tank.  Other popular chemical medias include: phosphate removers like Rowa Phos for removing phosphates, and carbon based media like Chemi-pure for removing silicates and other toxins just to name only two of them.  You can use chemical filtration in a filter, a media reactor, or passively in a sump provided the flow is correct for the media being used.  Just remember to check the manufacture’s recommendations for the proper use and follow those instructions.

Mechanical Filtration

There are a few different ways this is achieved.  Among the more common ways is to use a filter pre sponge on a filter or skimmer intake, using filter socks on a drain line or in a sump, or using sponges between two sections of a sump.  Some people do not even use any mechanical filter media in their marine aquariums at all.  As I have sumps on my marine tanks, I choose to use filter socks for mechanical filtration.  I prefer a 100 or 200 micron filter sock which seems to be fine enough to catch and filter out most partials from the water while not becoming clogged to fast.  Many people will use 200 and 300 micron filter socks to filter out all partials from their water, but I have found the finer material will clog up too quickly.  Mechanical filter media does require frequent cleaning in order to prevent nitrates from being produced and placed into the water. The exact frequency of the required cleaning will depend upon your set-up as this can vary greatly between different aquariums.

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