It is actually unfortunate that API suggests that 4 dose regimen. This is probably done in analogy to the use in Human medicine. For example, Azithromycin (an Erythromycin derivative) comes in a 5 pill blister pack (1 pill each day) to make sure that an effective dose is delivered to ALL body tissue, especially the lung. Since the antibiotic has to be transported to the tissue by the blood, this 5-day regiment is required to ascertain that all infectious bacteria are killed.
In the aquarium, however, an easier regiment can be used as this environment is very different. Normally, the water is circulated to allow gases and fertilizers to be distributed quickly. This also aids greatly in the distribution of any medicine.
Since cyanobacteria are not invasive, their treatment with Erythromycin is actually quite effective. As cyanobacteria grow mostly on surfaces (plants, substrate) and some also in the water column, the antibiotic Erythromycin can penetrate these blue-green bacteria quickly and kill them within a day.
It is recommended to use between 200 to 250 mg of Erythromycin for 10 gallons. It is imperative that this dose is used. When using antibiotics it is a must to use the maximum dose first (different from chemical reagents). Often LFS people suggest to use low amounts. Don't do half a dose, that's the wrong approach! When using the maximum amount of antibiotic, treatment of the aquarium infection with a single dose is sufficient. (Of course, multiple dosing can be used, but that is really not necessary).
Erythromycin at the recommended max. dose is a very safe antibiotic.
(It works by binding selectively to the 50S subunit of the bacterial ribosome and thus inhibits translation of bacterial mRNA that ultimately leads to cell death.) As only certain bacteria have this ribosome, Erythromycin has no effect on fish, plants, shrimp, or snails.