Coral are considered invert's and are highly sensitive to low amounts of copper, even more than shrimp/snails. Most test kits can't even detect the smallest of amounts that will cause problems in reef tanks.
It's kind of hard to believe though that you have any copper in your tank just because of that, but I guess possible. There has to be some other issue that you aren't aware of. I would hook that same pump up in the same bucket with the same salt and run an experiment with testing copper in that bucket's batch. If you say your tank tests something with copper, then a fresh batch under the same conditions should provide you with a positive answer since it will not be diluted w/50gals of other water. Infact, it should test off the charts in a 5gal mix if it's truly there. I would also mix up another batch in another bucket at the same time, and run a full out test on everything from Mg, Cal, dKh, everything you can.
Is the salt new salt? Is the water your water, or did you buy it from somewhere? Something has to be contaminated, and until I found out exactly what happened, I'd be leary of using anymore water/salt to do changes until I found the culprit as to not make a bad situation worse.
I am going to guess my zoas, shrooms, and polyps will be wiped out.
