An overdose of Excel will deplete oxygen. Your fish could go to the
surface but the bacteria could not. This is the reason behind the
loss of bacteria.
To help the fish out you can add some ordinary rock salt (not table
salt). This will prevent nitrite poisoning, the biggest killer in
freshwater. You can add up to 1 teaspoon per gallon for the fish
although this will be detrimental to the plants. Most plants can
tolerate 1 tablespoon per 10 gallons.
Ammonia is not that toxic to freshwater, depending on the pH. If
your pH is 7.0 or below the ammonia is nontoxic. If your pH is at
8.2 only 5% of what you are reading on the other test kit is toxic.
I packed a box for you but Fedex came before I got the label printed.
So it will go on Monday.