Was there any algae in the tank or excessive algae prior to your dosing the Excel ? Some Cyano Bacteria will release destructive/toxic enzymes after a die off. Some Rainbows are sensitive to Mycobacteria.

Other than that your chemistry looks great. It's a mystery ? Like Russ I've used Excel quite a bit, and even at twice the recommended dosage my Rainbows are fine. HTH. Prof M