You need to do more than a 40% change to get those nitrites in the safe zone. Keep doing water changes until they are undetectable. Then, do them whenever ammonia or nitrite reach .25 ppm or more.
The API stress coat is a good conditioner, but what it won't do that prime does is neutralize the ammonia and nitrite ions and make them non toxic for fish but still available for the beneficial bacteria to use.
And I agree 100% with jpappy -- do another water change now, you HAVE to get the nitrite concentration down.