How to Get Rid of Blue-Green Algae (Cyanobacteria) in Your Tank

Table of Contents

• What Is Blue-Green Algae?
• How to Identify Cyanobacteria
• What Causes Blue-Green Algae in Aquariums
• How to Remove Blue-Green Algae
• Erythromycin as a Last Resort
• Rebalancing Your Tank After Treatment
• Is Cyanobacteria Safe for Fish?
• Prevention Strategies
• Frequently Asked Questions

What Is Blue-Green Algae?

Despite its common name, blue-green algae is not actually algae at all. It is cyanobacteria — a photosynthetic bacterium that has existed on Earth for over three billion years. This distinction matters because it explains why standard algae treatments often fail against it and why antibiotic treatment works as a nuclear option.

Cyanobacteria thrive in warm, stagnant conditions, which makes Singapore aquariums particularly vulnerable. With ambient temperatures sitting comfortably between 28–32°C year-round, our tanks already provide the warmth these organisms love. Combine that with the nutrient-rich tap water from PUB and you have a recipe for a cyanobacteria bloom if other conditions are not kept in check.

At Gensou, we have dealt with cyanobacteria outbreaks in countless customer tanks over our 20-plus years in the aquascaping trade. The good news is that it is entirely treatable and preventable once you understand what drives it.

How to Identify Cyanobacteria

Cyanobacteria is one of the easier aquarium problems to identify thanks to several distinctive characteristics:

• Appearance: Slimy, sheet-like coating that spreads across substrate, rocks, plants and glass. It typically appears in shades of blue-green, dark green or even reddish-brown.
• Texture: Unlike most true algae, cyanobacteria peels off in sheets or mats. You can often lift an entire section away from the substrate in one piece.
• Smell: This is the dead giveaway. Cyanobacteria produces a distinctly foul, musty odour that many aquarists describe as swampy or earthy. If your tank suddenly smells unpleasant, check for blue-green patches.
• Growth speed: It can cover large areas surprisingly quickly, sometimes spreading visibly within 24–48 hours.

Blue-Green Algae vs Other Algae Types

For a broader overview of all algae types, see our guide on how to get rid of algae in your fish tank.

What Causes Blue-Green Algae in Aquariums

Understanding the root causes is essential for both treatment and long-term prevention. Cyanobacteria outbreaks are typically driven by a combination of factors:

Low Water Flow and Dead Spots

Cyanobacteria colonises areas with poor circulation first. Check the corners of your tank, behind hardscape and underneath plant leaves. If you notice BGA forming in the same spots repeatedly, your flow pattern needs adjustment.

The Low Nitrate Paradox

This catches many aquarists off guard. While most algae problems are associated with excess nutrients, cyanobacteria often appears in tanks with very low nitrate levels (below 5 ppm). Cyanobacteria can fix atmospheric nitrogen, giving it a competitive advantage in nitrogen-poor environments where true algae and plants struggle.

Dirty or Compacted Substrate

Old, anaerobic substrate creates perfect conditions for cyanobacteria. In Singapore’s warm climate, organic waste breaks down rapidly in the substrate, and without adequate circulation through the substrate layer, anaerobic pockets form that fuel BGA growth.

Excess Light with Insufficient Plant Mass

Tanks with strong lighting but sparse plant growth leave too much energy available for cyanobacteria to exploit. This is particularly common in newly set up aquascapes where plants have not yet established.

How to Remove Blue-Green Algae

Step 1: Manual Removal

Before anything else, physically remove as much cyanobacteria as you can. Use a siphon or gravel vacuum to suction the slimy mats directly. Because it peels off in sheets, you can often remove large quantities in a single water change session. Aim to remove 30–50% of the water during this process.

Step 2: The Three-Day Blackout

A complete blackout is one of the most effective non-chemical treatments:

1. Perform a thorough manual removal and 50% water change.
2. Turn off all aquarium lights and wrap the tank completely in black bin bags or thick towels. No light should enter.
3. Leave the blackout in place for 72 hours (three full days).
4. Do not feed your fish during this period (healthy fish tolerate this easily).
5. Keep your filter and air pump running throughout.
6. After 72 hours, perform another 50% water change and remove any remaining dead cyanobacteria.

In Singapore, ensure your tank temperature does not spike during the blackout. Without lights, temperatures should actually drop slightly, but monitor with a thermometer to be safe.

Step 3: Address the Root Cause

A blackout without addressing the underlying issue will only buy you time. After the blackout:

• Increase water circulation, especially in areas where BGA appeared.
• If nitrate levels are very low, consider dosing potassium nitrate (KNO3) to maintain 10–20 ppm.
• Vacuum and stir compacted substrate.
• Add more fast-growing plants to outcompete the cyanobacteria for resources.

Erythromycin as a Last Resort

Because cyanobacteria is a bacterium, it responds to antibiotic treatment. Erythromycin is the most commonly used option. However, we strongly recommend this only as a last resort after blackouts and manual removal have failed.

Why Caution Is Warranted

• Antibiotics can damage your beneficial bacteria colony, potentially causing an ammonia spike.
• Overuse of antibiotics in aquaria contributes to antibiotic resistance.
• It treats the symptom without addressing the cause — cyanobacteria will return if conditions remain favourable.

Dosing Protocol

If you must use erythromycin, dose at 200 mg per 40 litres of aquarium water. Treat for five days, performing a 25% water change before each re-dose. Monitor ammonia and nitrite levels daily throughout treatment. Have a reliable test kit on hand.

In Singapore, erythromycin for aquarium use can be difficult to source. Visit us at Gensou, 5 Everton Park, and we can advise you on suitable alternatives that are locally available.

Rebalancing Your Tank After Treatment

Whether you used a blackout or antibiotics, your tank will need some care in the weeks following treatment:

1. Monitor water parameters daily for at least two weeks. Watch ammonia, nitrite and nitrate closely.
2. Reduce feeding to every other day for the first week to minimise waste load.
3. Add beneficial bacteria supplements to help replenish any bacteria lost during treatment.
4. Increase plant density with fast-growing species like Hygrophila, Rotala or floating plants.
5. Maintain a consistent fertilisation schedule to keep nitrate levels in the 10–20 ppm range.

Is Cyanobacteria Safe for Fish?

Cyanobacteria is not harmless. While small patches may not cause immediate problems, significant blooms pose real risks:

• Cyanotoxins: Some cyanobacteria species produce toxins (microcystins and others) that can harm fish, invertebrates and even humans.
• Oxygen depletion: Large mats of cyanobacteria can create localised oxygen-depleted zones, especially at night when they switch from photosynthesis to respiration.
• Substrate suffocation: Cyanobacteria sheets can smother beneficial bacteria in the substrate and trap toxic gases like hydrogen sulphide.

Bottom-dwelling fish such as Corydoras and Kuhli Loaches are particularly at risk because they rest directly on the substrate where cyanobacteria tends to accumulate.

Prevention Strategies

Preventing cyanobacteria is far easier than treating it. Follow these guidelines for Singapore conditions:

Frequently Asked Questions

Can algae-eating fish or shrimp remove blue-green algae?

Unfortunately, no. Most algae eaters will not consume cyanobacteria because it tastes foul and can be toxic. Siamese Algae Eaters, Otocinclus, Amano Shrimp and Nerite Snails will all avoid it. You must rely on manual removal, blackouts or chemical treatment.

Will blue-green algae go away on its own?

It is extremely unlikely. Cyanobacteria tends to spread aggressively once established. Without intervention to remove it and correct the underlying causes, it will continue to worsen. The sooner you act, the easier the treatment will be.

Is it safe to put my hands in a tank with cyanobacteria?

While the risk is low for healthy individuals, some cyanobacteria species produce toxins that can cause skin irritation. We recommend wearing aquarium-safe gloves when performing manual removal, especially if you have any cuts or abrasions on your hands.

How long does a blackout take to kill cyanobacteria?

A 72-hour (three-day) blackout is the standard recommendation. Some stubborn cases may require a second blackout after a week of normal conditions. If two full blackouts fail to resolve the issue, it may be time to consider the erythromycin option or seek professional advice.

Get Professional Help with Cyanobacteria

Dealing with a stubborn cyanobacteria outbreak can be frustrating, but you do not have to tackle it alone. At Gensou, we have over 20 years of experience helping Singapore aquarists with algae and water quality issues. Visit us at 5 Everton Park for personalised advice, or explore our aquarium maintenance services if you would prefer a hands-off solution. Let us help you restore your tank to its best condition.

Related Reading

• How to Fix Blue-Green Algae on Rocks and Hardscape
• How to Fix Blue-Green Algae on Aquarium Sand
• How to Remove Blue-Green Algae From Aquarium Sand Substrate
• Green Spot Algae: What Causes It and How to Remove It
• How to Fix Green Dust Algae: The Film That Coats Everything