First step in any situation like this is an emergency water change(s).
Don't be afraid to do a 100% water change.
Obviously there is something toxic in the water that must be removed before you have any more losses.
After (or during) the water change remove the suspect item.
Get your tank back to the exact state it was in before the die-off began.
Then you can start the process of figuring out what might have caused the problem.
Check with the mfg of the trough to see if they use any additives or coatings.
Some plastic containers are coated to keep them from sticking together when they're stacked for storage/shipping.
Always wash, scrub and rinse well any unknown containers before using on your tank.
I once placed my hands in a tank after doing a round of water tests.
I must have spilled some test chemicals on my hands and not noticed.
Within 10 minutes I started to have a massive die-off.
The only thing that saved the remaining fish, inverts and corals was a 100% water change.
Thank God I'd recently hit the lfs
and had the containers of water onhand and ready to go.