New shrimp dying in new tank

[eternitybc]

So our tanke is 3 weeks old (since adding live rock/live sand, we waited three weeks to cycle before adding livestock). We added 2 cleaner shrimp and 2 crabs, so slowly grow the bacteria before we considered adding dish. One shrimp died on day 2, the second one made it to day 6 and looks like he's doing really bad. He's twictching, but the water current is floating him around/upside down.

I don't know what is killing them- there's no ammonia, the water stats look good- salinity / temp are matched and we drip acclimated for 3 hours.

There are 2 things I can think of... we used some old rock (dry rock from our last tank build) that we had outside for a year. I did a bleach water soak and rinsed it super well, and we let it dry 2 days and then mixed it with live rock from fish store. Is it possible something in the rock is killing the shrimps? Should I have acid etched them?

Also, we used black sand... I know it sounds silly but I googled and other people have had issues with black sand kiling shrimps. It supposed to be live black sand and it was new from the fish store so that seems unlikely but...?

 

[HumbleFish]

Was the old rock ever exposed to copper? If so, it might have absorbed some of it and then leached it back out.

The sand would only be an issue if metals/toxins were leaching out of it. I’ve read that some black sand is attracted to a magnet (e.g. Mag Float) which I’ve always found suspicious.