Okay—my job is really that of the village idiot, so forgive me while I ask some really stupid questions.
1. Do you need to put fish in that tank urgently for some reason? If yes... if No, then don't put anything else in that tank until you figure out what is going on.
2. You said it was a Redsea tank?? The same people that we see threads of blown-out seals, etc. (puts a pin on this thought)
3. You're picking fish from one tank in the same room to another tank in that room, so it's safe to assume it's not something in the air, correct?
4. If the fish were held in a tank there (if I understand correctly), when did you change the water in the old tank? Did you use the same source for the new tank? Same salt, etc.
Random Theories will abound, but keeping things simple, as they often tend to be, is either chemical or biological. Being an idiot, I thought I saw you tested for stray voltage, right?