Sorry to hear about your frog. Red leg is a very serious disease caused by a gram-negative bacteria called Aeromonas hydrophila. Usually it is fatal if not treated. In the early stages, you can use Maracyn, Maracyn Plus or tetracycline. Most of these products are found in a good pet store selling tropical fish supplies. You will also need to thoroughly clean the terrarium. Any other frogs in the terrarium must be removed and placed in quarantine (and treat if symptoms appear).