Fish Flex Forte (Cephalexin) 500 mg is a broad-spectrum antibiotic used in the treatment of a wide range of bacterial infections.Directions:add contents of one capsule (500 mg) into aquarium for each 20 gallons of water to be treated. Repeat 24 in hours. It is suggested that a partial water changed be made between treatments. While duration of treatment depends on type and severity of infection it is recommended that extended medications baths continue for a minimum of 5 days and not more than 10 days. Discontinue treatment if no improvement is noticed within 5 days.