Vibrio googles as a gram-negative bacillus. Bacilli come in 2 types, gram-negative and gram-positive [relates to how they stain on a slide---but also to how to treat them.] A broad-spectrum antibiotic is so called because it covers both. A gram-negative type antibiotic would possibly be more specific. I doubt it is a virus [subcellular infective agent] and suspect it is a bacillus [a complete cell lifeform]. The following is a statement on Maracyn-Two.
"Maracyn-Two® Powder Saltwater
is a fast-dissolving and quick acting antibiotic (Minocycline) powder recommended for the treatment of popeye, gill disease, fin and tail rot, dropsy, septicemia, secondary and internal infections. Minocycline is a broad-spectrum antibiotic which has been shown to be effective against a number of pathogenic bacteria associated with ornamental fish disease.
Benefit: Due to its unique properties, Maracyn-Two is absorbed by the fish through the skin. During the prescribed treatment, the concentration of Maracyn-Two in the fish fluids rises to a therapeutic level and is available to fight systemic infections susceptible to its action. Contains B complex vitamins that stimulate metabolism to improve appetite. Maracyn-Two may cloud the water but will not interfere with the biological filter. New activated carbon or charcoal and a 25% water change removes any cloudiness resulting from treatment.
Use: Maracyn-Two is primarily effective against gram-negative pathogenic bacterial diseases of fish. Symptoms of clamped fins, general listlessness, not eating or other unusual behavior can be caused by internal infections and treatment with Maracyn-Two would be indicated. Maracyn-Two is recommended for the treatment of Popeye, Gill disease, Fin rot and Tail rot, Dropsy (swollen body, protruding scales), Septicemia (bleeding or red streaks on body), secondary and internal infections. All suspected bacterial infections should be treated immediately. Powder should be administered into the tank (not the filter). The powder dissolves rapidly and circulates through the water. Use of a hospital tank is recommended. After treatment Maracyn-Two can be removed by water changes and fresh activated carbon.
Active Ingredients: Each packet contains 20mg Minocycline activity; 15.00mg Pantothenic acid, 8.60mg Riboflavin, 2.60mg Thiamine Mononitrate, 2.60mg Pyridoxine Hydrochloride."
Withdraw fish to quarantine to treat. do not use a carbon filter while treating. Use plain floss.
