sure looks like a melon to me.
CO2 injection certainly would not hurt any plants you might try to grow. CO2 is a requirement for all photosynthetic organisms. That being said... you may not see a huge difference in development and growth of a sword should you add CO2 injection... if you have very high lighting, then it would be a crucial upgrade... but barring that, I would say it would not be worth the investment for melon swords.