Same thing happened to me a few weeks ago. I did water changes on 3 tanks. nothing different to usual. About an hour later I'm watching my fish die. Lost an entire colony of Demasoni, A tank full of young and juvenile tangs, all my daffodils which were starting to breed nicely and their babies.
The big africans lay on the bottom for a while but then came right. Put that down to a big tank = large volume of water and lots of filtration.
I wonder if water services or whatever had just dosed or overdosed with chlorine and it was bad timing