Yes, 80 is high. I think the other readings were up to but have since cycled down.
Do a 20% water change ASP, make sure that your water has aged a bit...aerate for at least a few hours. So mix your water, bring your salinity up a little (this water change bring it up to 1.024) and then the next one bring it up to 1.025.
I don't believe that your nitrates killed your fish. maybe your featherduster.
Test, test and test some more. Each test will make you more comfortable with the test kit.
Turn this into fun......