So - the strange thing is that although we often recommend erythromycin in the water to treat bacterial eye infections or to reduce cyanobacteria, in food fisheries, it is most always used orally.
Then, erythromycin is formulated in different ways, and have different amounts of excipients/binders in it. That makes comparing dosages very difficult.
I just use Maracyn 1 and go with the label instructions....treat for 5 days.
If you are using pure Erythromycin, bulk powder, I would dose it at 12.5 mg/gallon, which is the value I use for large scale cyanobacteria control, and I know it is safe for a variety of fish.