Biological filters contain many bacteria themselves. These, too, can be affected and more than likely will be. Note that since live rock and live sand contains bacteria as well, the use of antibiotics will affect them as well. Cyanobacteria are photosynthetic. They release oxygen and uptake carbon dioxide, depending on the photoperiod and lighting conditions.
Cyanobacteria contain pigments.
The prevalence of one particular pigment ultimately determines the color of the algae. It should be noted that they are among the oldest form of organisms found on earth. Traces of cyanobacteria have been found in fossils dating from the early Precambrian period, some 3 billion years ago. They are extremely common, even today, and are found in fresh, brackish and salt water. They are present even on land in moist environments.