In conditions of low nutrients some organisms predominate over the others, for their better adaptive weapons. Very low phosphates appear to favor dinoflagellate blooms and, if the limiting is nitrate, this seems to favor cyanobacteria. Some strains of filamentous cyanobacteria are very similar to GHA in their visual appearance and are often confused; Lyngbya sp is one of them and is the most similar, being green, the other is Calothrix sp, but tends to be brown or more discolored.
If the "filamentous algae" in the rocks and sand are cyanobacteria, they usually resist Fluconazole, but are sensitive to Azithromycin.
When I used this medicine at a dose of 1 mg / liter in a single dose, I did not notice any effect on the animals in the aquarium and only noticed a slight rise in nitrites, with no ammonia or increased nitrates during the time of use, but still, for possible specificities and differences between each aquarium, whenever antibiotics are used the biological filter should be monitored closely, measuring ammonia, nitrite and nitrate throughout the treatment, to avoid surprises ...
As usual, when using medicines, the skimmer cup and the activated carbon or correlates should be removed so that the medicines are not withdrawn from the water before doing their work. The UV filter also has the ability to degrade the medication, so it should be turned off.
Against cyanobacteria, Azithromycin usually does its job in 4 to 5 days. After the cyan has disappeared, it is expected to be 02 days, before returning the skimmer cup and activated carbon to the line. In 24 to 48 hours the medicine has been withdrawn from the water column and after a partial water exchange of 25% to rebalance the parameters, the aquarium can return to its routine.
Best regards