Planted Tank Guru
Join Date: Jun 2004
Location: Baton Rouge, LA
One capful = 5ml of Excel.
That is the Seachem-recommended initial dose for a 10G, or 5x the daily dose.
However, I personally feel the recommended initial dose is too much, too fast, and have always avoided it. Instead I use only 1ml/10G daily, sometimes more if I'm treating for algae.
Now while I'm not sure if the Excel is to blame in your case, I do know this:
If a superchlorination event, rise in pH with possible ammonia toxicity, difference in TDS or temperature, lack of dissolved oxygen, or any other factor from your tapwater had been responsible, then the second and larger water change only two hours later would likely have killed the cardinals stone dead, not saved them.