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Saltwater Aquarium Filtration - Biological Filtration
Last updated on November 23rd, 2023
What is Biological Filtration for Saltwater Aquariums
Biological filtration is when bacteria, known as "beneficial bacteria" in the tank converts broken down waste from ammonia, nitrite, nitrate, etc and converts them to less toxic versions. For example, the conversion of ammonia to nitrite, and nitrite to nitrate. Bacteria converts each of these to the next, less toxic version. Some of this bacteria then eats the nitrates and is either eaten by coral or removed by the protein skimmer.
Though bacteria grows naturally after the tank is properly cycled, there are ways to help beneficial bacteria grow. By helping beneficial bacteria growth, the biological filtration in your saltwater tank will be stronger and handle more waste filtration.
Growing a Saltwater Biological Filter
The saltwater aquarium's biological filtration starts by growing in the initial nitrogen cycle when you set up a tank. Rather than repeat that information here, go read the article, How to Cycle a Saltwater Fish Tank.
After your tank's initial cycle is finished, your initial biological filter is established. This means that the bacteria responsible for converting ammonia, nitrite, and nitrate consumption, live on the surfaces of the rock and biological filtration media in your reef tank. But keep in mind, it is only build to the point where it handles what it currently has. This means no fish, no coral, no food. The biological filtration exists, but it's weak.
This is where we will look at how to strengthen the biological filtration using biological filter media.
Biological Filter Media
Biological filter media is really simple. Essentially you are just looking to provide a LOT of surface area for bacteria to grow and eat nitrogen sources. Seems simple. That's because it really is! There are some biological filter media that work better than others, so we will look at a few of those here.
Live Rock
Live rock is the most natural, and most common, biological filter media. Live rock is a porous rock found in the ocean where bacteria can form. So, before you ask, yes, it is just a rock where bacteria grows. The term "Live" in this case is referring to the living organisms, including the bacteria, that grows on and within the rock itself.
Unfortunately, one major drawback to live rock is that there is not a lot of surface area on live rock when compared to modern alternatives. This is why it is common to find a sump filled with rock. It requires a large amount of rock to provide the amount of surface area needed for the bacteria to grow. The live rock placed in the sump to reduce the clutter of the rock in the main tank. The rock in the main tank is part of this filtration though!
Though this is not always ideal, live rock provides a nice appearance and a familiar environment to most marine fish. Because of this, I would not scrap the idea of live rock too quickly, but if you're like me, perhaps you enjoy the clean, open look of a minimal aquascape filled with coral. Less rocks gives you more room for fish to swim and coral to grow, which means we have to look to other biological filter media for the reef aquarium to help with some of this filtration.
How Much Live Rock do I need?
The rule of thumb for "how much live rock per gallon do I need?" has traditionally been between 1 and 2 pounds of live rock per gallon of aquariums. With aquariums in the hundreds of gallons, this can take up space quickly in both the display and the sump.
Live Sand
Live sand, much like live rock, is another common biological filter found in the reef aquarium. There is a lot of debate about the depth of the sand, grain size, and other factors when discussing live sand. A takeaway of the live sand conversation is that sand beds over 2”, also known as deep sand beds, can lead to waste buildup within the sand and causing other problems like ammonia spikes if the sand bed be disturbed. On the other hand, a bare bottom tank, with no live sand, doesn't benefit from the added surface area provided by the sand, meaning nearly no area for the bacteria to grow.
Depending on the flow in your aquarium, it is possible you can't have a sand bed in your reef tank. If this is the case for you, there are a couple alternatives. First, you may use crushed coral substrates. This is thicker and doesn't move as much with flow. The second is to set up a refugium. The refugium is either a separate tank or section of the sump where you can add extra sand, rock, and macro algae to help reduce nutrients in the aquarium. If this whole sand thing isn't for you, the other option is to use an alternative biological filtration media. Let's look at some more alternatives.
Other Resources
Bio Balls and Bio Bale
Bio Balls and Bio Bales are a plastic biological filtration media, and alternative to sand and rock as a biological filtration media. These alternatives provide a large amount of surface area, generally much more than live rock, and can be cleaned very easily when needed. Bio balls and bio bale are some of the most common forms of biological filter media found in wet-dry filters.
Though wet-dry aquarium filters used to be very popular, we have evolved past this. These saltwater aquarium filters are rarely used anymore because there are even better alternatives such as the bio-bricks and bio plates. But if you're looking for a cleaner easily maintained options, these plastic biological filtration media are a good choice.
Bio Bricks, Bio Plates, Bi0 Spheres, ect.
A modern alternative to live rock as a biological filtration media are the Bio bricks and bio plates. The bio brick and bio plates are biological filter media blocks specially designed to provide the most surface area possible in a small space. These saltwater aquarium filter media are extremely porous and are well known in the reefing community for their ability to grow beneficial bacteria. These bio filter media are built for stacking, providing an easy way to add more without taking too much space. In fact, I use them as a stand for my protein skimmer, and in the bottom of my refugium.
Other Saltwater Aquarium Biological Filter Media
Of course, there are plenty of other options available. That being said, I find the bio bricks and plates to be the best option, but also take the most space. There are some other options available though that come in smaller options such as Seachem Matrix and Bio Cubes. These options can be used in a mesh bag or even as a base layer in your refugium for extra surface area. Though not falling in any of the above categories, there are other porous biological filter media types that may fit your needs better if you are limited by space. These biological aquarium filter media are essentially bio balls or small bio stones that are similar to the bio bricks and plates. All you need is to place this filter media in a media bag and toss it in your sump. Here are a few examples that I recommend looking into:
Adding Bacteria Manually
There are times where you may see spikes in ammonia and/or nitrites in your aquarium, usually after disturbing the sand bed or when something in the tank dies without you noticing. At this point, you may not have as much of a biological filter built up and you really need to handle the spike before it gets out of control. In cases like this, you may want to add beneficial bacteria manually by adding a bottle of bacteria culture such as Brightwell Aquatics MicroBacter7.
Another popular use for bottled beneficial bacteria is to cycle your new aquarium quicker. After adding something to start the cycle, such as food, the addition of these beneficial bacteria can actually help build a strong bacteria culture from the beginning. Though this may shorten the cycle itself, be careful not to rush things too much. You should still wait and monitor the aquarium for a week or two after the cycle appears to have completed. It is possible that not enough food was added when starting the cycle, meaning there was not enough bacteria built up to handle fish waste. I recommend using these products to supplement the cycle, just don't assume you can add them and be done.
How Much Biological Filtration Do I Need?
When it comes to picking a biological filter media, it all comes down to two things, surface area of the media and physical space to keep the filter media. It's best to use as much biological filter media as you can reasonably fit in the space and still easily maintain. Keep in mind though that you don't need sump chambers that are purely filter media. Try to work it in with the rest of the sump.
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About the Author
John Krenzer
John is a Software Engineer with a passion for saltwater aquariums, as well as the founder and president of Reef Stable. He started in the aquarium hobby as a child with a 20 gallon freshwater aquarium. His interest in aquarium life grew and in 2008, John set up his first saltwater aquarium.
Today, John maintains an over 300 gallon reef tank system, consisting of a 120g reef and a 210g reef. These large tanks are contained within the same system, sharing a sump as a means to reduce total maintenance and increase total water volume.
John writes articles for the blog as a means to learn about more reef aquarium topics. These articles act as a reference for the readers as well as himself. John updates these articles frequently to provide additional information or make corrections as new information becomes available.
If you would like to request an article, tank tour article, or to collaborate, let me know via the Contact Me Page!
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