What is blue-green slime in an aquarium?

Why this happens

Cyanobacteria are photosynthetic bacteria that can form smooth sheets with a distinctive odour. They spread quickly over substrate and plants where flow is poor or available nutrients and light favour them.

• Dead zones allow mats to establish.
• Organic waste and unstable nutrient balance can support blooms.
• Long lighting periods can accelerate spread.
• Some cyanobacteria can produce toxins, though risk cannot be judged visually.

Do not identify solely by colour; mats can be blue-green, dark green, brown or reddish. Slime texture, rapid sheet formation and odour are useful clues.

What to check first

• Does it peel away as a cohesive slimy sheet?
• Is there a strong earthy or unpleasant odour?
• Does it recur in low-flow areas?
• Are livestock or people exposed to the water?

Compare the result with the aquarium’s normal baseline, not only a generic target. Record the time, recent maintenance, new livestock, feeding changes, source-water details and any equipment fault. A short timeline often separates the original cause from secondary symptoms.

A practical action plan

1. 1. Limit exposureWear gloves, avoid aerosols and prevent ingestion by people or animals.
2. 2. Physically remove matsSiphon carefully during water changes and discard securely.
3. 3. Correct the environmentIncrease appropriate flow, remove waste and stabilise nutrients.
4. 4. Escalate persistent bloomsSeek specialist identification before using antimicrobial treatments.

How to interpret the response

A reduction in regrowth after improved flow and cleaning supports an environmental cause. A blackout may suppress photosynthesis but does not fix the underlying system alone.

Change one important variable at a time where welfare allows, then observe and retest. When several products or adjustments are made together, it becomes difficult to know what helped, what caused stress and whether the underlying problem remains.

What to avoid

Do not handle large mats bare-handed, allow pets to drink the water, scrub them into a fine suspension or use antibiotics without diagnosis and legal guidance.

When it is urgent

Livestock distress, human or pet exposure symptoms, a large surface bloom or drinking-water contamination concern requires urgent professional advice.

How to prevent it next time

Maintain circulation, avoid nutrient extremes, clean substrate zones and quarantine plants and hardscape from affected systems.

Keep enough records to recognise the aquarium’s normal behaviour, water chemistry and equipment performance. Trends are more useful than isolated numbers, and early correction is usually safer than a large emergency change.