Biofilm in aquariums

What is biofilm?

Biofilm is a colony of bacteria and fungi held together by a sticky matrix, growing on submerged surfaces. In tanks you’ll often spot it as a whitish, grey, or translucent film on new driftwood, leaves, or decor. It can look worrying but usually signals the tank is settling in.

Where does it show up?

New driftwood is the classic spot. Wood leaches sugars and organic compounds as it soaks, and bacteria feed on them. You might also see biofilm on new plants, seed pods, or any surface that offers food and attachment. It can appear within days of adding something new.

Is it harmful?

In most cases, no. Fish and invertebrates often eat it. Shrimp, snails, and some fish (e.g. otos, plecos) graze biofilm. It can be a useful food source. The main concern is looks — if it bothers you, you can remove it. If it spreads heavily and smells bad, that can point to excess organics or poor flow; address those rather than just wiping surfaces.

When to act

• Light film on wood or plants — Leave it. Grazers will clean it, or it will fade as the wood stops leaching.
• Thick, smelly slime — Siphon or scrub it off. Check feeding, water changes, and flow. See water parameters and overfeeding.
• On filter intake or equipment — Wipe or rinse during maintenance. Normal buildup.

How to reduce or remove it

Manual removal: wipe or scrub the surface, then siphon out loosened bits. Boiling wood before use can cut down initial leaching and biofilm. Adding grazers (shrimp, snails) helps keep it under control. If biofilm keeps coming back thick and fast, look at feeding, water change frequency, and whether you’re overstocked.

Quick takeaways

• Biofilm = bacteria and organics on surfaces; common on new wood and plants.
• Usually harmless; many fish and inverts eat it.
• Remove if thick or smelly; fix underlying causes (feeding, flow, maintenance).