I recently (5 days ago) double dosed my 125 gal with Excel (the initial dose indicated on the bottle) and some fish died.
I also dosed 1/2 tsp Epson Salt MGSO4 (magnesium sulfate) I bought it at the drug store and it also contains 7H2O.

I immediately did a 30% water change and 2 hrs later another 30% water change. and the rest of the fish survived.
The fish that died: 3 angelfish, 1 spotted pleco, 10 cardenal tetras.
Survivors: Neon Corydoras, clown pleco, 4 angelfish, 20 cardenal tetras

I plan to use Excel again but with half the initial dose. Seachem says that this product lasts for 24 hrs only. I think I'll dose at the regular regimen of once every other day, at the suggested dose.