There is always a possibility here, guys, of the seals holding any copper that may have been dosed. A similar scenario happened to a customer of the LFS
I frequent. He was sold a tank that was known to have copper medicine dosed in it. He has yet to keep any inverts alive in that thing for extended periods of time.
Now, I can't say for 100% certainty that is what is causing the problem (maybe he doesn't know what he is doing and it keeping the salinity to low perhaps, dunno), but it sure seems like too good of a coincidence to me.