Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Keep on doing water changes, and thoroughly vacuum the substrate. Just like you are rinsing the substrate when you first used it. You want to remove ALL the toxins in the tank, in the water, under the substrate, in the filter... Everywhere.
And yes, run some AC for a few days, then swap it out for new AC. After a week or so it will probably be OK to remove it.
If you think the nitrifying bacteria are dead and new livestock is on its way, then get some bacteria in a bottle. Look for Nitrospiros. Do not waste your money on anything else. The others (not labeled Nitrospira) have the wrong species of bacteria.
Get more test kits, or at least test strips.