How to Remove Blue-Green Slime Algae (Cyanobacteria) in Your Aquarium

Blue-green slime algae, better known as cyanobacteria, is one of the most frustrating problems in the hobby. It isn't actually algae — it's a photosynthetic bacteria that spreads fast, covering plants, substrate, and glass in a slimy green or bluish sheet.

What Is Cyanobacteria?

Cyanobacteria are ancient microorganisms that thrive in light- and nutrient-rich environments. In an aquarium they show up as slimy blue-green or dark green layers on substrate, plants, and decorations, often with air bubbles trapped underneath and a distinct earthy or swampy odor. Left untreated, it can suffocate plants and deplete oxygen.

Common Causes

Cyanobacteria usually appears when there's a nutrient or flow imbalance: low water circulation or dead zones, excess organic waste from uneaten food or decaying plants, high phosphate or nitrate levels, too much light (especially near windows), or irregular maintenance.

Manual Removal

Before treating chemically or biologically, remove as much as you can: siphon or gravel-vacuum the slime off substrate and décor, trim or remove heavily affected leaves, and clean glass with an algae pad or scraper. Repeat daily for several days, and avoid stirring the slime more than necessary — disturbing it releases spores that spread it faster.

Increase Flow and Oxygenation

Cyanobacteria thrives in stagnant water. A small powerhead or wave maker, adjusted filter output, and better surface agitation all make the tank less hospitable to it and encourage beneficial bacteria to compete instead.

Adjust Lighting

Limit lighting to 6–8 hours a day during treatment, reduce LED intensity by 25–50% if possible, and keep the tank out of direct sunlight. Once the bloom clears, ease the photoperiod back up to 8–10 hours to avoid triggering another outbreak.

Balance Nutrients with Live Plants

Cyanobacteria feeds on excess phosphates and nitrates, so fast-growing plants that absorb the same nutrients help starve it out. Hygrophila difformis, Limnophila aromatica Mini, Bacopa Myriophylloides, Hornwort, and Red Root Floater are all fast enough to make a real dent.

Water Changes

Change 30–40% of the water weekly during treatment, rinse filter sponges gently in tank water rather than tap, and test phosphates and nitrates, aiming to keep phosphates under 0.5 ppm and nitrates below 20 ppm.

Blackout Treatment

For stubborn outbreaks, cover the tank completely for 72 hours to block all light, keeping filter, heater, and air stone running. Uncover and do a 50% water change afterward. This starves the cyanobacteria of light while leaving plants largely unharmed.

A Good Starting Point

Our Hornwort is one of the fastest nutrient-absorbing plants available and a solid first move if cyanobacteria keeps coming back.

Preventing Future Outbreaks

Keep a consistent lighting schedule, feed lightly, add nutrient-absorbing plants, maintain good flow and oxygenation, and stick to a weekly maintenance routine. A balanced, well-planted tank resists cyanobacteria naturally.

Keep Reading

• Say Goodbye to Algae: The Best Ways to Clean Your Aquarium Glass
• Black Beard Algae (BBA): Causes, Prevention & the Most Effective Ways to Remove It