Doxycycline Hyclate Powder is a broad spectrum antibiotic derived from oxytetracycline.  Effective against both gram positive and gram negative bacterial ailments, such as Fin & Tail Rot, Septicemia and Mouth Rot.  It can also be used concurrently with Erythromycin to treat some forms of Columnaris.
 
Unlike tetracycline and oxytetracycline, Doxycycline Hyclate is unaffected by pH and remains active in high pH environments. If your tank is low pH (under 7.0) then any of the cycline medications can be effective.
 
Treatment:  250-500 mg (1/4 teaspoon) per 20 gallons with a 25% water change, daily for 10 days. 
 
10 gram jar treats approximately 225 gallons
100 gram jar treats approximately 2275 gallons