If you emptied and cleaned the tank then I doubt it's copper but it's still possible. Copper is a tricky thing and can still be around even when test kits say its not. Like TriggerFan said it can leech out of stuff over time. If it does end up being copper, hopefully the ICP test will tell you, there are a few things that will get rid of it. I've used Seachem Cuprisorb in the past, cheap and easy, 8 bucks treats something like 60 gallons. Stick it in a bag and shove it somewhere in your filter/sump like activated carbon and leave it there for a couple weeks. It could just be something like shock though. Your salt is a little low but not unbearable, but if the shrimp comes from somewhere with high salt it could be an issue. Shrimps are extra sensitive to salinity while crabs are pretty tough in general so its hard to compare the two. Try and raise your salinity a bit over time and do your best to match it and temperature when you acclimate.