Biofilm in the Aquarium: Friend and Foe
Biofilm coats every submerged surface in a tank. Most of it is the helpful backbone of a mature system; only some forms are a nuisance. Here is the difference.
Biofilm has a bad reputation among new aquarists, but it is one of the most important and unavoidable parts of a healthy tank. Understanding what it is, and which forms help versus which annoy, turns it from a worry into a tool.
What biofilm is
A biofilm is an aggregate of microorganisms whose cells adhere to each other and to a surface, embedded in a self-produced matrix of extracellular polymeric substances containing polysaccharides, proteins, lipids and DNA. Beyond bacteria, biofilms can include fungi, protozoa and algae. In an aquarium, a thin biofilm coats virtually every submerged surface, the glass, substrate, decor, driftwood and plants, as the tank matures.
The beneficial side
Most aquarium biofilm is helpful, and a mature tank depends on it.
- Biological filtration: the nitrifying bacteria that convert toxic ammonia and nitrite to safer nitrate live within biofilm on filter media and surfaces, so this microbial layer is the working heart of the nitrogen cycle.
- First food for tiny grazers: biofilm is a layer of nutritious bacteria, algae and other microorganisms, and it is a key early food that baby shrimp and fry graze on throughout the day.
- Sign of a stable system: a fine, even biofilm is normal in an established, balanced aquarium.
The nuisance side
Some biofilm-related growths are unsightly or signal an imbalance, usually too many available organics. These are managed rather than eliminated entirely.
| Form | What it is and what to do |
|---|---|
| Surface film / scum | Organics and microbes at the water surface; increase surface agitation |
| White film on new wood | Harmless temporary biofilm on fresh driftwood; leave it or let grazers eat it |
| Thick slimy sheets | Possible cyanobacteria (blue-green); treat as a separate issue |
| Cloudy water / bacterial bloom | Free-floating bacteria, often in a new tank; usually clears on its own |
How to tell harmless from problematic
A thin, patchy film that animals graze and that does not smother plants or surfaces is harmless and beneficial. Growth that becomes thick, sheet-like, foul-smelling, or that blankets plants and decor points to an excess of nutrients or organics. Manage the excess at its source: increase surface movement and circulation, keep a clean-up crew of shrimp and snails, and reduce overfeeding and waste so the system is not over-fertile.