This is an over the counter product - it does not require prescription.
Fish Flex (Cephalexin 250mg) is for use in aquariums for the treatment of bacterial activity. Cephalexin is a cephalosporin antibiotic and acts by inhibiting the synthesis of bacterial cell wall, leading to cell death. Cephalexin is active against both gram-positive and gram-negative bacteria. Resistance can develop among some gram-negative rods (e.g., E. coli).