The short answer
Green dust algae is a fine, soft green film that coats the aquarium glass — and sometimes hardscape — with a dusty, powdery layer you can wipe away with a finger. It’s a very common form of algae in tanks that are bright, young, or slightly out of balance on light and nutrients. It’s harmless to fish, but it clouds your view and can be stubborn if you fight it the wrong way.
What causes it
Like all algae, green dust is a symptom of imbalance between light, nutrients and CO2. It’s especially common in newer tanks that haven’t stabilised yet, and in tanks running a long or bright photoperiod. Excess nutrients from overfeeding or overstocking give it extra fuel. It settles on glass because that’s the brightest, most exposed surface in the tank.
How to clear it
The counter-intuitive part: constant wiping can keep it going. Green dust has a life cycle — if you wipe it just before it matures, the freed spores simply re-settle and start again.
- Let it mature, then remove. Leave the glass for one to two weeks so the film thickens and its spores settle out, then wipe it off in one go and do a large water change to export the debris. An algae scraper makes the final clean quick.
- Trim the light. Keep your photoperiod to 6–8 hours on a timer — long, bright lighting is the main trigger.
- Reduce nutrients. Feed less, don’t overstock, and keep up weekly 25–30% water changes.
Give a new tank time
If your tank is only a few weeks or months old, some green dust is normal while the system matures and biology settles. Keep the photoperiod modest, stay consistent with water changes, and it usually eases as the tank balances out. For the bigger picture, see how to prevent algae in a new tank and our full how to get rid of aquarium algae guide.