In a previous post, Maracyn was suggested as a Brown Algae control because it is a photosynthetic bacteria. They suggested that gram-negative meds would control it without harming the filter bacteria when gram-positive meds would. Maracyn is erythromycin... an antibiotic. I just don't know if tetracyclin is a gram-negative or -positive... and if it would have any effect on the Brown Algae.