Cyano Bacteria in Reef Tanks — Causes and Fixes

Cyanobacteria looks like red, purple, or black slime mats, often in low-flow corners with detritus. It is bacteria, not true algae, so “algae eaters” may ignore it. Causes include excess nutrients, poor flow, old bulbs, and accumulated waste behind rock.

Increase flow in dead zones, vacuum detritus during water changes, and reduce feeding. Some products knock back cyano temporarily — fix the underlying nutrients or it returns.

Cyano often signals maintenance drift, not bad luck. Photo the affected area and log nitrate, phosphate, and last water change date in ReefDiary when treating so you know what actually worked.