Biofilm is the thin, sometimes slimy layer that forms on surfaces in aquariums — especially on new driftwood, plant leaves, and decor. It can look ugly, so people often panic and scrub it off. But in many cases, biofilm is a normal part of a developing ecosystem. It’s a community of microorganisms breaking down organics, and it becomes a food source for small fish, fry, shrimp, and snails.
When biofilm is normal
- New driftwood: fresh wood often grows a pale film for weeks.
- New tanks: fresh systems develop surface films as biology stabilises.
- Heavily planted tanks: surfaces provide more area for biofilm to grow.
Why guppy keepers should care
For fry tanks, biofilm is actually a benefit. Fry graze on microorganisms all day, which supports steady growth even between feedings. Many breeders intentionally provide surfaces to encourage micro-life.
When biofilm is a warning sign
If biofilm is heavy and returns quickly after cleaning, it can indicate excess organic load: too much food, trapped debris, weak flow, or a filter that needs a gentle rinse. If it’s paired with cloudy water or bad smells, treat it as a maintenance signal.
What to do about it
- Leave it alone (often best): many tanks “grow out of it” as they mature.
- Improve circulation: gentle flow reduces dead spots where film builds.
- Control feeding: less surplus food means less film growth.
Biofilm isn’t automatically bad. In many guppy tanks, it’s part of a healthy ecosystem. The decision is simple: if fish are healthy and water is stable, it’s mostly cosmetic. If water quality is slipping, biofilm is one more clue to tighten your routine.
