Fish Flex Forte - Cephalexin - 500mg 100 CountFish Flex (cephalexin) is a broad spectrum antibiotic used in the treatment of a wide range of bacterial infections. It is effective against a number of pathogenic bacteria associated with ornamental fish. Available in 500 mg strengths in bottles of 100 capsules. Directions For Use Add contents of one capsule (500 mg) into aquarium for each 20 gallons of water to be treated. Repeat in 24 hours. It is suggested that a partial water change be made