I said nothing about nitrifying bacteria. That is also false that the antibiotic only harms cyanobacteria. That is just not how erythromycin works. The concern is the microbiome of the fish and coral. Antibiotics can greatly impact the health of organisms as the microbiome of said organisms becomes damaged. This isn't even mentioning the antibiotic resistance crisis occurring, of which pouring antibiotics into an aquarium can't be helping. Antibiotics are actively harmful and should only ever be used as a last resort when all else fails.
PS if anyone is interested in how antibiotics impact the human gut microbiome, I have colleagues that run a lab on interactions between humans and the gut microbiome.