Calothrix Algae: Identification and How to Get Rid of It

Hektor Jorgo By Hektor Jorgo Published Updated 4 min read
Dark tufted cyanobacteria growth on aquarium hardscape, often confused with black beard algae

Quick Facts

What It Is
A genus of filamentous cyanobacteria (blue-green algae), not a true algae
Appearance
Dark green to black, hair-like or tufted growths on hardscape, plant leaves, and slow-flow areas
Often Confused With
Black beard algae (BBA) — but Calothrix is bacteria and responds differently to treatment
Smell Test
Cyanobacteria often has a musty/earthy smell when disturbed; true algae generally doesn't
Common Triggers
Low flow/dead spots, nutrient imbalances (often low nitrate relative to phosphate), organic buildup
Manual Removal
Can be siphoned out, but tends to return quickly if flow and nutrients aren't addressed
Chemical Treatments
Erythromycin-based products can work but carry risks to the biofilter and are usually a last resort
Best Long-Term Fix
Improve flow to dead spots, correct nutrient imbalance, and manually remove during the adjustment period

If you've found dark, hair-like, slightly slimy tufts in your aquarium and assumed it was an unusually stubborn case of black beard algae, you're not alone — Calothrix and similar cyanobacteria are among the most commonly misidentified growths in the hobby. The distinction matters, though: Calothrix isn't algae at all, and the fixes that work for true algae don't always work for it.

Direct Answer: It's Cyanobacteria, Not True Algae

Calothrix is a genus of cyanobacteria — often called "blue-green algae" despite not being algae in a biological sense. It typically appears as dark green to black, hair-like or tufted growths on hardscape, substrate, and plant leaves, frequently in areas with low water flow. The "blue-green algae" name comes from the broader group of cyanobacteria, which can also appear as slimy sheets in shades of green, black, red, or even purple depending on species and conditions.

Because it photosynthesizes and forms similar-looking mats to true algae, it's commonly lumped in with "algae problems" — but its biology, causes, and treatment overlap only partially with true algae, which is covered more broadly in our algae guide.

Identifying Calothrix vs. Black Beard Algae

The two are confused often enough that it's worth a direct comparison:

Feature Calothrix (cyanobacteria) Black Beard Algae (true algae)
Texture Slimy, often peels off in sheets Coarse, bristle-like, clings tightly
Smell when disturbed Often musty/earthy Generally none
Typical location Low-flow areas, substrate, decor Plant edges, decor, filter outflow areas
Removal difficulty Can be siphoned, but returns if flow/nutrients aren't fixed Notoriously hard to remove even with algae-eaters
Response to flow changes Often improves with better circulation Less directly linked to flow

If you're unsure, the "peels off in a sheet and smells off" test is one of the more reliable quick checks for cyanobacteria over true algae.

What Causes It to Take Hold

Calothrix and related cyanobacteria favor low-flow, organically-rich areas — spots where debris settles and isn't regularly disturbed by filter circulation. Common locations include behind large decor pieces, in substrate corners away from filter outflow, and on slow-growing plants like anubias and java fern that don't get much direct flow across their leaves — though the green spot and film algae more commonly seen on those plants is a different (and far more common) issue than cyanobacteria specifically. Beyond flow, nutrient imbalances are frequently implicated — particularly situations where nitrate is low (often because fast-growing plants are consuming it efficiently) while phosphate remains relatively higher from feeding and fertilizers. This imbalance can favor cyanobacteria over plants and true algae in affected areas.

Improving flow to dead spots — covered in general terms in our canister filter inlet/outlet placement guide — addresses one major contributing factor directly.

Treatment Options: Flow, Nutrients, and (Sometimes) Chemical Treatment

  1. Improve circulation to affected areas — repositioning filter outflow, adding a small circulation pump, or rearranging decor to eliminate dead spots is often the single most effective long-term change.
  2. Address nutrient imbalance — if nitrate is consistently very low while phosphate isn't, adjusting fertilization or feeding to bring the ratio closer to balance can reduce the conditions cyanobacteria favors.
  3. Manual removal during the adjustment period — siphoning out visible mats (rather than just scraping, which can spread fragments) reduces the existing population while flow/nutrient changes take effect.
  4. Erythromycin-based treatments — an antibiotic sometimes used against cyanobacteria, but it can also harm the beneficial bacteria in your biofilter, risking an ammonia spike. This is generally considered a last resort after flow and nutrient fixes haven't worked, and requires close water quality monitoring during and after treatment.
  5. Blackout periods — some keepers use a multi-day total blackout (no light) combined with manual removal as a more drastic reset, though this stresses plants as well and is best combined with addressing the underlying flow/nutrient cause rather than used repeatedly as a standalone fix.

Quick Reference

  • Calothrix is cyanobacteria, not true algae, despite the "blue-green algae" name
  • Dark, slimy, hair-like tufts that peel off in sheets and smell musty point to cyanobacteria
  • Low-flow dead spots and nitrate/phosphate imbalances are the main causes
  • Improving circulation to affected areas is often the most effective fix
  • Manual siphoning (not just scraping) reduces the existing population
  • Erythromycin-based treatments work but risk the biofilter — treat as a last resort
  • Address the underlying cause, or removal alone won't be permanent

Frequently Asked Questions

How do I know if it's Calothrix or black beard algae?

The two are easy to confuse at a glance because both form dark, hair-like tufts, but there are a few practical differences. Calothrix and other cyanobacteria often have a slimy texture and can be peeled away in sheets, sometimes with a musty or earthy smell when disturbed. Black beard algae (BBA) is a true algae with a coarser, bristle-like texture that's harder to remove and doesn't typically have the same smell. If scraping or siphoning a patch produces a slimy mat that lifts off relatively easily and smells distinctly 'off,' cyanobacteria like Calothrix is the more likely culprit; if it's tough, wiry, and clings stubbornly to leaves and hardscape, BBA is more likely. Our general algae guide covers BBA and other true algae types for comparison.

What causes Calothrix to start growing?

Cyanobacteria generally takes hold in low-flow areas where organic matter accumulates — behind decor, in substrate corners with poor circulation, or anywhere debris settles without being swept away by filter flow. It's also frequently associated with nutrient imbalances, particularly low nitrate relative to phosphate, which can happen in tanks that are heavily planted (using up nitrate quickly) but still accumulating phosphate from food and fertilizers. Our canister filter placement guide covers how flow patterns affect dead spots, which is directly relevant to where cyanobacteria tends to establish.

Will a UV sterilizer get rid of Calothrix?

UV sterilizers are designed primarily for free-floating microorganisms passing through the water column — algae spores, some pathogens, and free-floating bacteria — and have limited effect on cyanobacteria that's already established as a film or mat on surfaces, since that growth isn't in the water column where the UV light reaches it. A UV unit might help reduce free-floating cyanobacteria cells before they settle and colonize a surface, but it's not a substitute for addressing flow and nutrient causes once a visible mat has formed.

Is Calothrix harmful to fish or plants?

Generally, established cyanobacteria mats are more of an aesthetic and tank-management problem than an acute toxicity risk in most home aquarium situations, though some cyanobacteria can produce compounds that are best not left to accumulate indefinitely, and a mat covering plant leaves can block light and smother slower-growing plants like anubias. The bigger practical concern is usually that cyanobacteria mats indicate a dead-flow or nutrient-imbalance area that's worth correcting regardless of the cyanobacteria itself, since the same conditions tend to cause other issues (debris buildup, localized water quality problems) over time.

Sources & Further Reading

  1. Cyanobacteria Identification & Control — Practical Fishkeeping
  2. Planted Tank Algae Discussion — The Planted Tank Forum
Hektor Jorgo

About the Author: Hektor Jorgo

Co-Founder & Marine Biologist

Hektor is a co-founder of Sea Life Planet and has kept reef and freshwater aquariums for over 15 years. He holds a background in marine biology and focuses on species care accuracy, water chemistry, and tank husbandry.

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