Fix Doxy 100 (Doxycycline) 100 MG 60CTFix DOXY 100 (Doxycycline Hyclate) is an antibiotic that exerts a bactericidal action on gram positive and some gram negative bacteria in Fix. This Fix antibiotic is useful for control of some common bacterial diseases in Fix, including Septicemia, Mouth Rot, and Fin and Tail Rot. Directions: Add 1 dose (100 mg) into aquarium for every 15 gallons of water to be treated. Repeat in 24 hours. It is suggested that a partial water change be made between