It's odd that it was all tangs, and I'm stumped big time. You didn't mess with the sand, no toxin was introduced as everything else is fine, fish are quarintined, and no power outage. Best of luck and I hope you get this figured out.
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Nothing wet my coral buety angel, six line wrasse , 2 chromis , 1 damsel and all my tangs died. Not just my tangs but many others