Biological Filtration in Public Aquariums: The Science

Every stable public aquarium exhibit depends on an invisible workforce: colonies of nitrifying bacteria living on biofilter media, continuously converting the ammonia excreted by fish and invertebrates into progressively less toxic compounds. Unlike mechanical filtration, which can be inspected and cleaned on demand, biological filtration is a living system — and understanding its actual microbial behavior, not just its function, is essential to operating an aquarium LSS reliably.

The Microbial Reality Inside a Biofilter

It is tempting to think of an aquarium biofilter as a static piece of equipment once it has "matured," but peer-reviewed research shows the underlying microbial community is more dynamic than that framing suggests. A study published in PLOS One examining the temporal and spatial stability of ammonia-oxidizing archaea and bacteria in aquarium biofilters found that community composition can shift over time and vary by location within the same filter system — meaning a biofilter's nitrifying capacity is not a fixed, one-time achievement but an ongoing biological process worth monitoring (PLOS One, "Temporal and Spatial Stability of Ammonia-Oxidizing Archaea and Bacteria in Aquarium Biofilters"). This finding is significant for LSS operators because it means two functionally similar biofilters, even in similar exhibits, may host different microbial populations performing the same job.

The Two-Step Process: Ammonia to Nitrite to Nitrate

Regardless of which specific organisms are present, aquarium biofiltration follows the same two-step nitrification chemistry documented across the aquaculture and environmental microbiology literature: ammonia-oxidizing bacteria and archaea first convert ammonia into nitrite, and a separate group of nitrite-oxidizing bacteria then convert that nitrite into nitrate (Preena et al., 2021, Reviews in Aquaculture). Because ammonia toxicity is directly tied to pH and temperature, and because the proportion of the more dangerous unionized ammonia form rises as both increase, exhibit water chemistry directly shapes how much biofiltration capacity a given collection actually requires (University of Florida IFAS Extension, FA031: Ammonia in Aquatic Systems).

Media Surface Area and Sizing Principles

Nitrifying bacteria live as a biofilm on the surface of filter media, meaning the total available surface area — not the physical volume of media alone — governs how much nitrifying capacity a biofilter can support. Engineering research rating fixed-film nitrifying biofilters in recirculating systems found that removal performance is directly tied to surface area combined with water chemistry conditions such as pH, alkalinity, and dissolved oxygen — the same underlying principles that govern industrial aquaculture biofilter sizing apply directly to large-scale aquarium LSS design, since the microbiology is identical even though the application differs (ScienceDirect, "Rating fixed film nitrifying biofilters used in recirculating aquaculture systems"). This cross-disciplinary overlap is one reason LSS engineers frequently draw on aquaculture engineering literature when sizing biological filtration for exhibit volumes that can range from a few thousand liters to several million.

Why Disruption Risk Is an Operational Concern

Because nitrification depends on a living, functioning microbial community, anything that disrupts that community — a sudden drop in dissolved oxygen, a disinfectant residue reaching the biofilter, or a rapid temperature swing — can measurably reduce nitrifying capacity, sometimes for days or weeks while the community recovers. This is why aquarium LSS protocols typically treat biofilter protection as a standing operational priority rather than a one-time design consideration.