Blue-Green Algae (BGA)

Clinical Identification

Cyanobacteria forms a continuous, slimy, gelatinous blanket that rapidly smothers substrates, plants, and hardscape.

• Coloration and Texture: The biofilm exhibits a striking blue-green, emerald, or occasionally deep purple hue. The texture is distinctly mucilaginous; it holds together like a flimsy sheet when lifted but disintegrates easily under heavy flow.
• Olfactory Indicators: Cyanobacteria produces geosmin and 2-methylisoborneol (MIB), imparting a highly distinct, pungent "earthy" or "swampy" odor to the aquarium water, which is often detectable even before the visual bloom.
• Gas Entrapment: During peak photoperiods, the thick mat heavily traps oxygen bubbles generated via photosynthesis, leading to floating sheets of slime.

Document Blue-Green Algae (BGA) with close and wide images, check whether the material detaches, and measure light, nutrients, flow and organic load over time.

Compare Blue-Green Algae (BGA) with other algae, cyanobacteria, biofilm and mineral deposits using texture, location and microscopy when useful.

Bacteriological Profile and Physiology

Cyanobacteria, colloquially misnomered as "blue-green algae," are not true algae (eukaryotes) but rather highly advanced, photosynthetic prokaryotes. They represent one of the oldest lineages on Earth. Structurally, they lack membrane-bound organelles and chloroplasts; instead, their photosynthetic machinery is embedded directly into the folds of their cell membrane (thylakoids).

One of the most biologically devastating advantages of Cyanobacteria is the presence of heterocysts—specialized, thick-walled cells capable of nitrogen fixation. When ambient nitrate levels fall to zero, Cyanobacteria can enzymatically cleave atmospheric nitrogen gas (N2) dissolved in the water column to synthesize their own bioavailable nitrogen, completely bypassing the nitrogen cycle limitations that constrain higher plants and true algae.

The listed conditions are associations to verify in the individual aquarium, not universal causes or operating thresholds.

Pathophysiological Triggers

1. Nitrate (NO3) Depletion: The most common trigger in planted aquaria is a severely skewed Redfield Ratio, specifically profound nitrogen deficiency. When NO3 reaches 0 ppm while phosphates remain available, eukaryotic plants arrest, whereas nitrogen-fixing Cyanobacteria thrive.
2. Stagnant Hydrology: Low water velocity creates localized zones of anoxia and detrital accumulation at the substrate level, providing an undisturbed physical matrix for the cyanobacterial sheets to anchor and spread.
3. Accumulated Organic Detritus: Heavy mulm and decaying organic matter create massive biological oxygen demand (BOD), weakening plant vitality and providing complex carbon molecules that some mixotrophic cyanobacterial strains can exploit.

Increase gas exchange immediately, siphon the mat without dispersing it, and check oxygen, ammonia and nitrite; avoid antibiotics or biocides without identification and risk assessment.

Manage Blue-Green Algae (BGA) with physical removal, targeted pruning and documented correction of light, flow, nutrients or organic load. Any product requires its official label and compatibility checks for species, plants, invertebrates and biofilter; no dosage is provided here.

Prevent new Blue-Green Algae (BGA) growth with regular maintenance, healthy plant biomass, proportionate lighting and gradual measurement-based changes rather than universal targets.

Act urgently if the bloom coincides with distressed animals, a major oxygen drop, suspected toxins, or detectable ammonia/nitrite.