Types of Filtration in Freshwater Aquariums #
The dominant filtration solution in freshwater aquariums consists of devices based on a static, submerged biological filter bed. Water flows through a stationary medium, typically made of ceramic elements or sponge. In smaller tanks, internal or hang-on-back filters are most commonly used. In contrast, canister filters are the preferred choice for medium-sized aquariums. Large tanks are more often equipped with custom filtration solutions.
The Role of Microorganisms in Biological Filtration #
The functioning of this type of filter is based on the microbiological activity of organisms living on the surface of the filtration medium. These microorganisms secrete a polysaccharide, gelatinous substance rich in enzymes that break down impurities. In addition to bacteria and fungi, this substance also harbors micro-predators that feed on them. This process creates a specific micro-ecosystem known as a biofilm.
From the perspective of aquarium water purification, the biofilm that forms on solid submerged elements is far more significant than bacterioplankton (free-floating bacteria in the water column).
Disadvantages of Static Biological Filter Beds #
The static biological filter bed is undoubtedly a well-proven and effective solution. However, like any system, it has certain characteristics that may be considered suboptimal in specific circumstances.
From the perspective of a process engineer or water technology specialist, the key characteristic of this type of filtration medium is the gradation of conditions within it. This occurs both along the water flow through the filter bed and as water penetrates into the interior of a porous element (such as a piece of ceramic). Water carrying impurities— which simultaneously serve as substrates for enzymatic reactions conducted by microorganisms— undergoes purification progressively as it flows through the filtration medium.
This means that the availability of essential nutrients for microorganisms gradually decreases while the concentration of reaction byproducts (e.g., CO₂, NO₃⁻, H⁺) increases. Only in the first layer of the filter bed do conditions closely resemble those in the aquarium— with full availability of organic compounds, oxygen, and stable pH. A similar variation occurs as water penetrates deeper into porous elements.
In reality, only the outermost layer of the filtration medium provides optimal conditions for aerobic decomposition of impurities. However, this does not mean that the rest of the filter bed is inactive— rather, the availability of substrates progressively declines. On the other hand, these changes create conditions more favorable for other types of microorganisms. For example, as oxygen levels decrease while nitrate concentrations rise (due to the activity of aerobic nitrifying bacteria), conditions become increasingly suitable for microorganisms capable of denitrification or nitrate respiration.
The Impact of Nitrate Reduction in Different Aquarium Setups #
This process ultimately leads to a reduction in nitrate (NO₃⁻) concentration. In planted aquariums, it is common for the availability of nitrogen compounds, such as nitrates, to become a limiting factor for plant growth. To counter this, aquarists often supplement the tank with fertilizers containing nitrogen.
In such aquariums, the nitrate-reducing effect of static biological filter beds may be considered suboptimal. However, in plant-free tanks, this feature is undoubtedly beneficial. It helps to maintain water quality by reducing excess nitrogen compounds.
Alternative: Moving Bed Biofilm Reactor (MBBR) #
An alternative solution, used for several decades in industrial applications and increasingly adopted in aquaristics, is the moving bed biofilm reactor (MBBR). In this system, microorganisms form a biofilm on the surface of specially designed plastic carriers. Inside the reactor, water is continuously mixed, causing the carriers to move freely throughout its volume.
This movement ensures that all microorganisms have equal and consistent access to substrates, creating optimal conditions for aerobic decomposition of impurities. While processes like denitrification may still occur within the biofilm due to microenvironment variability, the conditions for such processes are significantly less favorable compared to static filter beds.
The Biofilm Renewal Effect #
Another fascinating feature of moving bed biofilm reactors (MBBR) is the constant renewal of the biofilm. The continuous movement and collisions of the plastic carriers cause gradual abrasion of the biofilm, which stimulates its regeneration.
This is a crucial advantage. Decades of industrial experience with this technology have demonstrated that MBBR systems offer remarkable efficiency compared to traditional alternatives. The continuous rejuvenation of the biofilm not only enhances water purification efficiency but also boosts the system’s adaptability.
This means that both the species composition and the enzymatic activity of microorganisms (i.e., the type and quantity of enzymes secreted into the polysaccharide matrix) adjust more effectively and rapidly to the composition of pollutants entering the reactor.
In simple terms: MBBR bioreactors have been able to break down substances that other microorganism-based filtration solutions could not handle.
Additionally, their higher efficiency allows for the use of smaller bioreactors while maintaining the same level of water purification. Another key advantage is their greater resistance to toxic substances, thanks to their enhanced adaptability. The continuous renewal of the biofilm enables it to adjust rapidly to changing conditions, making MBBR systems more resilient and effective in fluctuating environments.
Advantages of MBBR Reactor in Aquaristics #
When using moving bed biofilm reactors in aquaristics, two additional benefits can be highlighted:
- Lower installation costs: The prices of canister filters and ceramic media are continuously rising, whereas plastic carriers used in MBBR systems remain significantly cheaper than ceramic media. Additionally, the reactor itself, which consists of a simple plastic casing and a pump, is competitively priced compared to conventional canister filters.
- Lower operating costs: The moving bed filter medium offers much lower hydraulic resistance than static ceramic or sponge-based filter media. This means that a lower-power pump is sufficient to maintain water circulation, leading to reduced electricity consumption and lower energy bills.
Disadvantages of Moving Bed Biofilm Reactors #
Despite their many advantages, moving bed biofilm reactors (MBBR) also have some potential drawbacks. These include:
- Lack of reliable guidelines for reactor sizing – There is no clear standard for determining the optimal number of plastic carriers, required water flow, or the appropriate reactor size for a given aquarium. This makes DIY setups challenging and often requires experimentation.
- Noise from moving carriers and air bubbles – The constant collisions of plastic carriers produce a characteristic rattling sound. Additionally, the most common mixing method—using air bubbles—creates noise from bubbling water and the operation of an air pump, which may be bothersome in quiet environments.
- Limited availability of ready-made solutions for medium and large aquariums – Currently, most commercially available MBBR filters are designed as small internal filters, making them suitable only for small tanks. There are no widely available plug-and-play solutions for larger aquariums, requiring custom-built setups.
- Water cloudiness caused by biofilm abrasion – As the biofilm constantly regenerates, small particles can be released into the water, leading to temporary cloudiness, which may be undesirable in display aquariums.
Experiment: Using a Moving Bed Biofilm Reactor in a Planted Aquarium #
Encouraged by the highly positive results of this technology in industrial applications I have encountered in my professional career, I decided to test whether it is possible to mitigate potential drawbacks and successfully implement moving bed filtration in a medium-sized planted aquarium.
Previously, I filtered my 375-liter aquarium using three canister filters filled with a total of 15 liters of ceramic media. During a complete tank reset, I replaced the entire filtration system with a new setup based exclusively on a moving bed biofilm reactor.
Selection of the Reactor and Media #
I decided to use approximately 4 liters of plastic carriers. Based on the commonly accepted guideline among aquarists that suggests using about 1 liter of media per 100 liters of aquarium capacity.
Industrial reactor design guidelines indicate that the carriers can make up as much as 60% of the reactor’s volume. From this, I estimated that the minimum required reactor size for my 375-liter aquarium should be slightly under 7 liters.
A practical solution turned out to be Aquaforest AF130 reactors, which have a capacity of 5 liters each. To ensure sufficient processing capacity, I used two units in a series configuration, providing a total volume of 10 liters. With 4 liters of carriers, this results in a 40% fill ratio, leaving room for additional media if necessary— for example, if the fish stocking level increases.
Noise Reduction by Eliminating Aeration #
To minimize noise, I decided to mix the plastic carriers using only water flow, eliminating the need for additional aeration.
From my previous experiments using an oxygen meter, I knew that even during the most critical nighttime period, when plants are not photosynthesizing, the oxygen concentration in my aquarium never drops below approximately 5 mg/dm³, while during the day, it reaches close to 100% saturation.
Since aerobic bacteria responsible for water purification remain highly efficient at oxygen concentrations of around 2 mg/dm³, I concluded that additional aeration was unnecessary. This approach significantly reduced noise by eliminating the sound of air bubbles and the operation of an air pump while maintaining effective biological filtration.
Oxygenating the water beyond this concentration does not significantly improve filtration efficiency. Combining this fact with the stable oxygen levels in my aquarium, I concluded that additional aeration is unnecessary for the proper functioning of the filter. It is sufficient to supply the reactor with well-oxygenated water from the aquarium.
The only remaining challenge was ensuring constant carrier movement without using air bubbles. The columnar design of the AF130 reactor naturally supports mixing through water flow. I achieved this by reversing the water inlet and outlet connections, meaning I supplied water to the “out” port instead of the standard inlet.
This modification forced water to flow from top to bottom, pushing the carriers downward and maintaining continuous movement. Additionally, I made a small internal adjustment by installing a PVC elbow fitting inside the reactor. This induced a spiral downward flow, which significantly improved carrier circulation and ensured optimal biofilm exposure to nutrients and oxygen.
Using a Pump with Adjustable Flow Rate #
This setup allowed me to fine-tune the water flow— strong enough to keep the carriers in motion but not so powerful that they would stick to the bottom of the reactor.
In practice, the pump I used has a maximum flow rate of 5000 l/h, which turned out to be slightly oversized. To achieve optimal performance, I set it to only 35% power, which corresponds to an energy consumption of approximately 12W— a significant improvement compared to the 60W consumed by my previous filtration system.
Eliminating air-driven mixing noticeably reduced the noise level. While the reactors are not completely silent, the emitted sound is quiet enough not to be disruptive, even at night. Of course, this perception is subjective, but in my experience, the difference is significant, making the system much more comfortable for home use.
Eliminating Mechanical Filtration and Its Impact on Water Clarity #
To assess whether abrasion of the biofilm from the moving carriers would actually cause water cloudiness, I completely removed mechanical filtration from the system. This included removing the sponges that come standard with the AF130 reactors. The only mechanical filtration present is a small surface skimmer that keeps the water surface clean.
After several weeks of operation, I have not observed any visible cloudiness in the water.
The rate of biofilm growth on the plastic carriers is not as rapid as in wastewater treatment systems. This is because the concentration of organic pollutants, which serve as a food source for microorganisms, is much lower in an aquarium than in wastewater.
With limited nutrient availability, microorganisms prioritize using resources for survival and energy production rather than for rapid biomass growth. Only when excess nutrients are available does the biofilm develop more aggressively.
If, over time, I find that adding mechanical filtration is necessary, it will be easy to integrate into the system without major modifications.
Accelerating Aquarium Maturation #
Wanting to ease and speed up the maturation process of the tank, I took care to pre-condition the newly purchased filter media. About 1.5 months before the planned setup, I placed them inside a canister filter running in a well-established aquarium. This allowed me to have media already covered with a biofilm of living microorganisms at the moment of setting up the new aquarium.
Of course, I also took additional steps to support the maturation process. I left a thin layer of the old substrate, rich in beneficial microorganisms, and used supplements to accelerate the maturation process.
Summary of the Experiment #
Thanks to these measures, the aquarium started up almost without any issues. Within just a few days, I observed a clear increase in nitrate levels, indicating that the nitrifying bacteria were functioning properly. The water remains clear, and the water parameters are stable (NH₄⁺ 0, NO₂⁻ 0, NO₃⁻ 5 mg/dm³, PO₄³⁻ 0.5 mg/dm³, pH 6.6). The fish show no signs of stress, and the tank is developing as expected.
This article was created in collaboration with the Podwodne Światy Aqua Test channel. You can find the video of this experiment here.