Cyanobacteria (blue-green algae) in a planted tank: causes and control

A bacterium that looks like an alga

What aquarists call blue-green algae, smear algae or slime algae is not an alga at all. Cyanobacteria are bacteria: their cells have no true nucleus, but like plants they photosynthesize ^[1]. The US Environmental Protection Agency describes them as photosynthetic organisms common in fresh, brackish and marine water, many of which can fix nitrogen from the atmosphere ^[2].

In a planted tank they show up as dark green, blue-green or almost black coats on leaves, decoration and substrate. The coats feel slimy, lift off in sheets, and have a pungent, earthy or foul smell; the filamentous genus Oscillatoria is a frequent culprit ^[1]. Aquasabi notes that small populations, for example along the line where substrate meets the front glass, are normal and no cause for concern ^[1].

Why it appears

The sources name three drivers:

• Feeding. Tropica says the blue-black coatings formed by blue-green algae usually appear only with excessive feeding, and that feeding less usually makes them disappear ^[3]. Its general advice names overfeeding, or too many fish, as the number one cause of algae problems ^[4].
• Phosphate high, nitrate low. Aquasabi reports that outbreaks in planted tanks were often linked to a highly elevated phosphate level, sometimes combined with a nitrate deficiency ^[1]. Our reading of why (an inference, not a sourced finding): nitrogen-fixing cyanobacteria can make their own nitrogen from air ^[2], so a tank short of nitrate but rich in phosphate removes the advantage plants and true algae would otherwise have.
• A young biofilm. Aquasabi also sees outbreaks during the cycling phase, when the other microorganisms that compete for nutrients are not yet established ^[1].

Aquasabi’s suggested nutrient targets for a fertilized planted tank are about 20–30 mg/L CO2, 10–25 mg/L nitrate, 5–10 mg/L potassium, 0.1–1 mg/L phosphate and more than 10 mg/L magnesium, with roughly 50 % of the water changed each week ^[1]. The nitrogen, phosphorus and potassium guide puts those numbers next to the EI and PPS-Pro ranges.

What to do

1. Remove it. Cyanobacteria do not attach firmly, so small patches siphon off easily ^[1].
2. Cut feeding and change water. Tropica’s own remedy is less intense feeding ^[3]; water changes are, in its words, one of the most efficient measures against algae because they stop surplus nutrients accumulating ^[4].
3. Correct a nitrate gap. If you test and find phosphate high and nitrate near zero, raise nitrate rather than adding more of everything. The fertilizer dosing calculator gives the grams of potassium nitrate for a target.
4. Black out a large outbreak. Aquasabi recommends a blackout for heavier infestations ^[1]: complete darkness for several days with aeration, CO2 and fertilizer off, and a large water change afterward ^[5]. A blackout removes the growth, not the cause ^[5].

Aquasabi also mentions algicides and hydrogen peroxide treatments as first aid ^[1]. This site does not cover chemical treatments; if you use one, follow the product label.

The potassium treatment, in numbers

Aquasabi describes a “potassium treatment” for planted tanks: dose a single-ingredient potassium fertilizer until potassium is around 30 mg/L, hold it there for about a week with nitrate around 20 mg/L, then do a maximum water change and return to normal dosing ^[1]. Aquasabi presents this as a measure that “very probably” works through a surge in plant growth, not as a tested result.

The arithmetic is simple. Potassium sulfate (K2SO4, 174.25 g/mol) is 44.9 % potassium by mass ^[6], so raising potassium by 20 mg/L in 100 L of water takes 2,000 mg ÷ 0.449 ≈ 4.5 g. Without a potassium test you do not know the starting level, so treat the 30 mg/L as an approximate ceiling rather than a measured value.

When it is not cyanobacteria

A brown film that wipes off easily and appears in a new tank is more likely to be diatoms. Green, cloudy water without mats is a green-water bloom. Dense dark tufts that resist wiping are typically black beard algae.