hmm, that's the same model wet dry i have. i took it straight from the shrinkwrap, and hooked it up. with no problems. it COULD have been contaminates in it, but is unlikely, though certainly possible.
what type of water did you use to make the new sw? ro/di or straight tap water.
when i read the post my initial thought was chlorine/chloramine posioning. but you may have used ro water, whcih would rule that out.
did you test all your other parameters? ammonia, nitrite, nitrate etc.. what about the ph of the added water.
but it may be easy to rule out the water. the model 75, holds only about 5 gallons at most, what percentage is that compared to your tank? i would think it's prrobabl minimal.
whatever it was, hopefully you'll be able tofigure it out before any other fish die. it seems odd too, because damsels are so hardy. i've seen poeple dump damsels into a 1 hor old tank, fileld with straight tap water and no conditioning or anything.