I find it hard to accept that a single double dose of Excel will kill any fish, let alone the number that you experienced. I have done that several times, with no effect at all on the fish that was noticeable. MgSO4 is very hydroscopic so what we get always has some water absorbed in the crystals. That is all the 7H2O is - water incorporated in the crystals.

Did you add a dechlorinator when you did the water change before the double dose of Excel? If not, that is far, far more likely to be the cause of the fish deaths.