Blue-green slime (cyanobacteria): Identification & Treatment
Sheet-like blue-green or reddish slime with a distinct earthy smell — cyanobacteria, not true algae.
Blue-green slime (cyanobacteria)
Blue-green slime (cyanobacteria) is one of the algae types Studio Scaped tracks in freshwater aquariums. This guide covers how to tell it apart from the types it is most often confused with, what conditions bring it on, the readings worth taking before you treat, and how to stop it returning. In most tanks the deciding factor is light versus what your plants can actually use.
How to identify blue-green slime (cyanobacteria)
Sheet-like blue-green or reddish slime with a distinct earthy smell — cyanobacteria, not true algae.
Compare the photograph above with your own tank before treating: several algae types look alike at a glance but respond to completely different corrections, and treating the wrong one wastes weeks. If you are unsure, the full identification guide sets the common types side by side.
Why it appears
Algae is a symptom rather than the disease. In this case the usual trigger is low nitrate with excess organics and poor flow in dead zones. The full list of causes worth ruling out:
- Low nitrate with excess organics and poor flow in dead zones
- Detritus accumulation in substrate or filter media
- Stagnant areas with low oxygen
- Often follows over-cleaning that removes too much beneficial bacteria at once
What to measure, and what the readings mean
| What to measure | Typical target | What an off reading points to |
|---|---|---|
| Nitrate (NO₃⁻) | 5–20 ppm planted · under 10 ppm reef | Climbing nitrate signals more waste going in than water changes and plants export. Near zero in a planted tank starves plants and favours algae. |
| Light intensity and photoperiod | 6–8 hours for most planted tanks | Light beyond what your plants can use is left over for algae. Reduce duration before intensity — it is the easier variable to control. |
| Filtration and flow | 4× tank volume per hour freshwater · 6×+ reef | Low turnover leaves dead spots where detritus settles. Check working flow rather than the box rating, which assumes clean media and no head height. |
| Tank age / maturity | past initial cycling | New tanks pass through predictable blooms as biofilm and bacteria establish. Some of this resolves with time rather than intervention. |
| Bio-load | under 75% of the tank guide | The waste your stock actually produces. Plan for adult size — juveniles grow into their bio-load. |
Take these readings before changing anything. Treating without a baseline leaves you unable to tell which correction worked, and algae problems usually have more than one contributing cause.
Treatment & fixes
- Remove sheets manually and siphon affected substrate
- Improve flow to eliminate dead zones
- Increase water-change frequency and vacuum detritus
- Temporarily reduce light; cyanobacteria often retreats when organics are controlled
- Antibacterial treatments exist but fix hygiene and flow first
Keeping it from returning
- Reduce photoperiod before intensity; light your plants cannot use is left over for algae
- Keep flow reaching the whole tank — dead spots are where this establishes first
- Dose to your plant mass, not to zero — starving the tank starves plants first
- Quarantine new plants and hardscape, which is how most algae arrives in the first place
Once corrected, blue-green slime (cyanobacteria) usually takes two to four weeks to visibly recede — existing growth does not disappear when the cause does, so remove what you can manually and judge success by whether new growth appears.
Related guides
- Dinoflagellates — often confused with this type
- Brown diatoms — often confused with this type
- Every algae type, side by side — for identification
- Lighting and spectrum
Frequently asked questions
How long before blue-green slime (cyanobacteria) clears up?
Expect two to four weeks after the cause is corrected. Remove sheets manually and siphon affected substrate Existing growth generally has to be removed by hand — correcting conditions stops it spreading rather than dissolving what is already there.
Will algae eaters deal with it for me?
Grazers help hold a corrected tank clean; they rarely fix an uncorrected one. Nothing grazes cyanobacteria — it is a bacterium rather than an alga. Manual removal, flow and blackout are the tools that work.
Should I strip nutrients to starve it out?
Usually backfires. Plants stall before algae does, and the nutrients they stop using become available to the algae instead. Balance light, CO₂ and dosing to your plant mass rather than cutting fertilisation to zero.
