Very sorry man my heart sinks when i hear this.
We just went through this with keepinfishes p14... It didnt make sense that it was ok in first tank, then ok in my tanks for 3 weeks, (and my whole system never showed any signs of sickness anytime after) and then all in a sudden sick in keepins tank...
I would guess that the new rays became stressed for some reason and got sick, and then yes spread to the other rays... But i wouldnt necessarily blame the previous owner. But they prob should have been quarantined first to adjust to your water chemestry before mixing with your personal rays.