Many guppy keepers notice a brown film appear on the glass, substrate, or decor, sometimes within days of setting up a tank. It looks dirty, and it can make people feel like they’re failing at maintenance. In most cases, it’s part of a new aquarium finding balance — and it’s either biofilm (bacteria colonies) or diatom algae (a common brown algae that feeds on available nutrients and silicates).
Biofilm vs diatoms: how to tell
- Biofilm: feels slippery, often appears on wood and surfaces, and can look clear-to-brown.
- Diatoms: look like fine brown dust, often wipe off easily, and show up on glass and substrate.
Why it appears so fast
- New tanks: microbial life is establishing, so films are common.
- Extra nutrients: overfeeding and trapped debris accelerates it.
- Light and time: longer light hours can push algae growth.
What to do (without overreacting)
- wipe glass during weekly maintenance
- reduce feeding if debris is building
- keep water changes consistent and moderate
- avoid “reset” cleaning that destabilises biology
Is it harmful to guppies?
Usually not. In fact, some biofilm is a natural food source for shrimp and fry. The issue is more about aesthetics and what it signals. If your tank has thick brown buildup plus cloudy water and high nitrates, it suggests your input (food/waste) is higher than your routine can handle.
Brown film is often a phase. Keep your maintenance steady, keep feeding clean, and it usually fades as the tank matures and stabilises.
