[bertoni]

Erythromycin is an antibiotic, not an oxidizer. It's more likely to be a bit of a reducing agent, actually. I've never measured oxygen in conjunction with dosing an antibiotic, but I doubt there's be much change unless there was a very large amount of decaying cyanobacteria remnants in the water column.

That said, adding oxidizers can lead to a drop the oxygen level in a tank if the oxidizers kill enough organisms.