How to Fix Cyanobacteria in Your Aquarium: Blue-Green Algae Removal
This fix cyanobacteria aquarium guide provides a comprehensive approach to identifying, treating, and preventing one of the most frustrating problems in planted tanks. At Gensou Aquascaping, based at 5 Everton Park in Singapore, we have dealt with cyanobacteria outbreaks in numerous client tanks and our own setups, and we can confirm that it is entirely manageable with the right strategy.
What Is Cyanobacteria?
Despite being commonly called blue-green algae, cyanobacteria are not true algae at all. They are photosynthetic bacteria, among the oldest organisms on Earth, dating back billions of years. In aquariums, they typically appear as a slimy, sheet-like coating that can be green, blue-green, dark red, or even black. One of the most distinctive characteristics of cyanobacteria is its strong, unpleasant musty odour. The slime can cover substrate, plants, hardscape, and glass, and it spreads rapidly if left unchecked.
Identifying Cyanobacteria
Cyanobacteria is relatively easy to distinguish from true algae. It forms smooth, cohesive sheets or mats that peel away from surfaces in large pieces when disturbed, unlike filamentous algae which tends to break apart. The texture is slimy and slippery rather than rough or fuzzy. When removed, it often releases a characteristic earthy or swampy smell. Cyanobacteria also tends to trap gas bubbles beneath its surface, causing it to lift from the substrate in patches. If you can peel a slimy green sheet from your substrate or plants, you are almost certainly dealing with cyanobacteria.
Common Causes in Aquariums
Cyanobacteria outbreaks are typically triggered by a combination of factors rather than a single cause. Low nitrate levels are one of the most consistent contributing factors, as cyanobacteria can fix atmospheric nitrogen, giving them a competitive advantage when dissolved nitrogen is scarce. Poor water circulation, organic waste buildup, and an imbalanced nutrient regime also promote growth. New tank setups with immature biological filtration are particularly vulnerable. Excessive lighting relative to plant mass and nutrient availability can further tip the balance in favour of cyanobacteria.
Manual Removal and Water Changes
The first step in combating cyanobacteria is manual removal. Use a gravel vacuum or siphon to remove as much of the slime as possible during water changes. Lift sheets of cyanobacteria from plant leaves, hardscape, and substrate surfaces. Perform large water changes of 50 per cent or more to dilute any toxins produced by the bacteria and to reset water chemistry. Manual removal alone rarely eliminates an outbreak entirely, but it reduces the bacterial load and creates better conditions for other treatment methods to work effectively.
Addressing the Root Causes
Treating the symptoms without addressing the underlying causes will result in recurring outbreaks. Test your water for nitrate and ensure levels are at least 5 to 10 parts per million. If nitrate levels are consistently near zero, increase dosing of potassium nitrate or adjust your fertilisation regime. Improve water circulation by repositioning filter outlets or adding a circulation pump to eliminate dead spots where cyanobacteria tend to establish first. Reduce the photoperiod to six to seven hours temporarily and ensure organic waste is not accumulating in the substrate.
Blackout Treatment
A complete blackout is an effective non-chemical treatment for cyanobacteria. Cover the entire aquarium with thick blankets or cardboard to block all light for three to four consecutive days. Turn off any aquarium lighting and ensure no ambient light reaches the tank. Cyanobacteria rely on photosynthesis and are significantly weakened by prolonged darkness. Plants will survive a three to four-day blackout with minimal damage, though some sensitive species may lose a few leaves. After the blackout, perform a large water change and resume normal lighting with a reduced photoperiod initially.
Antibiotic Treatment
Because cyanobacteria are bacteria rather than algae, they respond to antibacterial treatments. Erythromycin is the most commonly used antibiotic for cyanobacteria in aquariums, dosed at approximately 200 milligrams per 40 litres over a three-day course. This treatment is highly effective and typically eliminates cyanobacteria within days. However, antibiotics can also affect beneficial filter bacteria, so monitor ammonia and nitrite levels closely during and after treatment. In Singapore, erythromycin may be available through veterinary or aquarium specialty channels. Always use antibiotics as a last resort and in conjunction with addressing root causes.
Preventing Future Outbreaks
Prevention is built on maintaining balanced aquarium conditions. Keep nitrate levels adequate through consistent fertilisation, maintain good water circulation throughout the tank, perform regular substrate maintenance, and avoid overfeeding. Ensuring a healthy plant mass that outcompetes opportunistic organisms is the best long-term defence. If you are battling a persistent cyanobacteria problem or want advice on optimising your tank conditions to prevent outbreaks, the team at Gensou Aquascaping can assess your setup and recommend targeted solutions for Singapore’s local water conditions.
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