Excel:
1x is enough/plenty, no need for 2x, which killed the fish.
50-80% would have been fine for a WC.
Not 100%.

I think some folks got a bit crazed with Excel and in their impatience, suggested to folks with BBA or Hair algae to bomb the plants at the sake of the fish's health.

Excel will kill your fish and I think that's more than likely when combined with the 100% daily water changes, the temp might have helped, but was not the entire story.

Regards,
Tom Barr