Every aquarium has some algae; a little on the glass and rocks is a sign of a living system. It becomes a problem when it spreads faster than you can clean it. Algae always point to an imbalance of light, nutrients and CO₂, so scraping it off without fixing the cause only buys a few days.
Identify the type
- Brown dust (diatoms): common in new tanks during the first weeks. Wipes off easily and usually disappears on its own as the tank matures.
- Green spots on glass and slow leaves: too much light for the plant mass, or low phosphate in planted tanks.
- Green hair or thread algae: excess light with unstable CO₂ or nutrients.
- Black beard / brush algae: dark tufts on edges and wood; linked to fluctuating CO₂ and poor flow.
- Green water: free-floating algae after a nutrient spike, often in bright tanks or ponds.
- Blue-green slime (cyanobacteria): smelly sheets on the substrate; linked to low nitrate, poor flow and dirty areas. It is actually a bacterium.
Fix the cause
Light
- Cut the photoperiod to 6–7 hours with a timer.
- Don’t place the tank in direct sunlight.
- If you have strong light without CO₂, dim it.
Nutrients
- Feed less and remove uneaten food.
- Keep up weekly water changes to keep nitrate and phosphate in range.
- Test your tap water; phosphate and nitrate often come from there.
- In planted tanks, algae also appear when plants lack nutrients. A plant that can’t grow leaves the nutrients to algae, so see nutrient deficiencies.
CO₂ and flow
- Keep CO₂ stable: switch it on an hour before the lights and off an hour before they go off.
- Make sure there are no dead spots; gentle flow everywhere helps plants and discourages black beard algae and cyanobacteria.
Help from clean-up crews
Amano shrimp, otocinclus and nerite snails graze many soft algae. They help, but they don’t replace fixing the balance.
Special cases
- Green water: a UV sterilizer clears it within days; a few days of darkness plus water changes also works.
- Cyanobacteria: siphon it out, increase flow, and raise nitrate slightly if it is near zero.
Fishtify tracks the nitrate and phosphate levels behind most algae problems and gives light recommendations for your tank type and plants.