Given what transpired, I suspect that a massive algae die-off or something toxic from the sand bed that sucked out the oxygen in the water; therefore, fish couldn’t breathe and died? Again, I’m not a fish medic - just sharing possible causes if that’s all you did.
I would clean up the tank and if you suspect any parasites (doesn’t seem to be) go fallow and slowly put fish back in later?
Good luck with your recovery.