I don't believe Corydoras are particularly susceptible to excel. Over gassing CO2 or over dosing excel are both dangerous to all fauna. There are many cases on the web about fauna killed due to over dosage of excel, not just cories.
I disagree slightly with what the Seachem Rep. said about Corydoras paleatus being less efficient in low oxygen environments than the other fish. Corydoras spp. gulps at the water surface as an adaptation against low oxygen levels in murky waters. Although Corydoras paleatus originates from rivers and lakes, this adaptation is embedded to the genes of all Corydoras spp..
When oxygen levels are low, you can spot cories dashing to the water surface constantly for air. They will not lie sideways, upside-down, right-side up, etc waiting for themselves to suffocate. It is not just cories, but other fish go up to the surface and gasp for air as well when oxygen is insufficient.
I've experienced bristlenose plecos acting in the same manner when over gassed with CO2. They enter a paralyzed state and drop sideways, upside-down, and motionless but exposure to oxygen quickly revives them. Whether it's because catfish are scaleless or some other reason, I don't know.
I do agree that the cory died from low oxygen by entering a paralyzed state and unable to gulp air from the surface, but not because Corydoras paleatus are inferior at coping with low oxygen levels.
You dose 5 mL excel as the initial dosage when you do a major water change OR never dosed excel previously before. Then dose 1 mL excel per 10 gallons daily or every other day. Multiply by 2-3 as over dosage ratio.
VICTOR PIMP #53 SGT-500-40-4F-2S-DK