Biological Filtration The term biological filtration refers to the process of lie bacteria converting organic waste in the form of ammonia and converting it into nitrates 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 nitrates and is a more natural way of providing biological filtration. There are many other benefits of using live rock in a marine aquarium that I won’t get into now. Live rock is my preferred method for biological filtration and I always recommend it.
2. Filters with Biological Filter Media or Biological Filter Media in a Sump: These are commonly used in a marine aquarium, but my least preferred method. While it certainly provides a lot of surface areas for bacteria to grow and can by very effective at converting ammonia and nitrates, 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. Many hobbyists do take this approach with no long term negative effects, just some extra weekly maintenance.

I have always had the best results using live rock for biological filtration. Times that I had tried using a canister filter with biological filter media, I struggled to obtain close to the same results as the live rock would always provide me. However, I have very high personal standards when it comes to the water quality in my marine aquarium so to each their own. 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 more commonly used chemical filter media is the use of carbon to remove toxins from the water when or 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 and carbon based media like Chemi-pure. 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 proper use and follow those instructions. I always prefer to have at least some chemical media in my set-ups as a precautionary measure in case any toxins are released into my aquarium. That is why I will use a carbon based media passively in the first compartment of my sump. In the past, I also used phosphate removing media in a media reactor so I could control the flow to match the manufacturer’s recommendations. Although I have not used any phosphate removers in a long time, I always have carbon based media in my marine set-ups. 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-ups as this can vary greatly between different aquariums. Nitrate and Phosphate Removal This is the one area that hobbyist can spend the most effort on. Higher levels of nitrates and phosphates can lead to problems such as nuisance algae growth or even stressing your fish and corals if the levels get high enough. This can be done in a few different ways – live rock, algae scrubbers, macro algae, carbon dosing, water changes, deep sand beds and refugiums.

1. Live Rock will not only hold the bacteria that processes ammonia and nitrates, but it will also hold bacteria that also eats nitrates and converts nitrogen into a nitrogen gas which is released into the atmosphere. This is one of the more natural ways to reduce the nitrates produced by your set-up.
2. Algae Scrubbers are designed to hold/grow algae allowing this algae growth to naturally remove nitrates and phosphates from the water. You are basically using flow and high levels of lower spectrum lighting to grow algae. Although an algae scrubber can also provide some biological filtration, their primary benefit is the removal of nitrates and phosphates. This too is a very safe and proven way to remove both nitrates and phosphate from your set-up. Some research suggests that one of the benefits of algae in your tank is that the algae can consume or filter out some unwanted trace elements out of the water (like you would expect from chemical filtration). The algae also provides a food source for most pods and will put some good trace elements back into the water.
3. Macro Algae grown in your set-up can remove a lot of nitrates and phosphates from the water. Although a macro algae can also provide some biological filtration, their primary benefit is the removal of nitrates and phosphates. This is a very common approach for many hobbyist to use macro algae, like cheato (the common name for Chaetomorpha algae), in a sump or refugium. Growing mangroves in your set-up is also said to work just as well as macro algae, but I do not have firsthand experience with it. I have obtained really good results with a few different types of macro algae grown in a sump.
4. Carbon Dosing can also be very effective; however, it is not a good fit for most people who are new to the hobby. I have tried a few different approaches to carbon dosing and found it to have great results, pretty fast as well. I now prefer to use more natural methods to removing nitrates and phosphates from my marine aquariums. There are some real risks to carbon dosing so make sure you do your homework first before tying it. I had stopped carbon dosing some time ago in favor of more natural methods that are a lot less risk to your set-up.
5. Deep Sand Beds are another approach if you want to explore other methods of removing nitrates and phosphates from the water.
6. Refugiums can also be an effective means to remove nitrates and phosphates from your set-up. This really is not a unique form of nutrient removal, but it does allow you to include a combination of some of the above mentioned methods in a partially enclosed environment. The more commonly found approaches used include deep sand beds, live rock and macro algae. This can be set-up as a compartment in your sump or as a separate tank/container plumbed into your set-up.
7. One of the simplest and easiest ways to reduce nitrates and phosphates in an aquarium is a complete water change. An adequately maintained marine aquarium with a good weekly water change routine should have minimal nitrates and phosphates in the water.

As with most aspects to filtration, more is usually better. That is why I will always use a combination of a few different ways to remove nitrates and phosphates from my water. Protein Skimmers They remove organic waste / proteins (solid and otherwise) from the water before these substances have a chance to break down and eventually result in nitrates. As a skimmer does this mechanically, it will not nicely fit into one of the above mentioned categories. That is why I did not mention a skimmer under any of the above headings as it can be included in more than one category. This is only one of the many reasons that I will always have a skimmer on my marine aquariums. I like to have a skimmer that is rated for a higher water volume as compared to the set-up I am placing it on. By following these multiple approaches to proper filtration in your marine tank, your water parameters for ammonia, nitrites, nitrates and phosphates should all remain undetectable by test kits. Overall, it’s recommended to hire a knowledgeable aquarium service company if you are new to the hobby, lacking knowledge on proper tank maintenance or just don’t have the time to dedicate to your marine tank. Not properly caring for your aquarium will result in costing you more money in the end, which is why good service and maintenance is a must.