Did you check the pH of where they were compared to your own tank? Did you add them slowly? Was the temperature similar?
By ruby shark, do you mean
Epalzeorhynchos frenatum, aka rainbow or red finned shark?
If so, these are better to be kept a single one to a tank as they are aggressive towards their own species - especially as they get older. I see you added them one at a time which I finally realised you meant you added the 2nd after the 1st died (I assume)
They need a large tank too, at least 3ft in length.