Biofilm is microscopic communities of bacteria, fungi, and protozoa bonded to surfaces by extracellular polymeric substance (EPS). Where it forms: glass, decor, plant leaves, filter media, substrate - everywhere. Initial biofilm establishes within 24-48 hours; mature biofilm takes 4-8 weeks. Why it matters: 1) biofilm hosts the bulk of the nitrifying bacteria (over half of "biological filtration" lives in biofilm, not just media). 2) Shrimp + snails + otocinclus + many fry actively graze biofilm - it's their primary food. 3) Surface biofilm appears as a dull haze on glass; deeper biofilm coats decor in a soft slime. When to disturb: only the visible top layer during weekly cleaning - never strip every surface, you'll trigger a cycle. "Mature tank": a tank with established biofilm + 6+ months runtime is dramatically more stable + supports finicky species (discus, apistos, sensitive shrimp).
Reviewed by the Fast Aquatics husbandry team · Updated May 2026
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