I am pretty sure the main ingredient is erythromycin.

Cyano bacteria is gram positive bacteria and erthromycin is highly effective in treating gram positive bacteria. Fortunately, the bacteria that we rely on to deal with nutrients in our reefs (primarily Nitrosomonas and Nitrobacter bacteria) are gram negative and are largely unaffected by erythromycin. Thus, using it isn't devastating to our biological filtration.

For a little more info about what "gram negative vs gram positive" means read below:

Gram positive and gram negative is one of the ways that microbiologists classify bacteria. The procedure is based on the chemical and physical properties of their cell walls. It refers to how a bacteria reacts to a gram stain. If it takes the initial stain, it will be purple and be considered gram positive. If it doesn't take the initial stain, it will be pink and gram negative. The difference is the outer casing of the bacteria. A gram positive bacteria will have a thick layer of protein that the stain can penetrate. A gram negative bacteria has an outer membrane covering a thin layer of protein on the outside. The outer membrane prevents the initial stain from penetrating.