If you have the capability to rescue them, I deffinately would. Those fish deserve a better home than what they have now. I don't know if hith can be cured when it is that severe, but I have heard some say that it can be cured from minor cases. Regardless, you would have to dose aquarium salt for who knows how long to reverse its progress. I would still get them though. It would be a good feeling knowing you rescued those fish from a bad home.