Can i put another turtle in my tank?

General rule of thumb for aquatic turtles is 10 gallons per inch of carapace length... more if you have multiple turtles, and this will NOT guarantee that there will be no squabbling. Turtles are NOT social animals... the only time they seek "friends" is during breeding season, and unwanted company will be seen as a territorial threat... not to mention potential food aggression...