I have used antibiotics labeled for agricultural crops successfully on Phalaenopis for bacterial leaf spot. They should only be used as a last resort. Using antibiotics labeled for fish would be a formula for disaster.
Crown rot is normally caused by fungi, against which antibiotics are ineffective. I would respectfully suggest that improved culture, particularly improved air movement, temperature management, and irrigation scheduling should reduce problems. If these persist or cultural conditions cannot be changed, use a fungicide and be sure to use the correct protective gear when applying it. Stuart Miller Ornamental Horticulture Chattanooga State Technical Community College
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