Assuming you haven't done any treatment whatsoever, I would start QTing the sick fish before they die. Don't underestimate ich even if you think it's not the main issue.
and ammonia level would lead to think that also NO2
are low, but make sure to check that as well. Are you sure the oxygen level is fine?
Finally, depending on tank size, you might want to perform a more robust water change.
That was my $0.02. Now wait for the professionals