Bumpy - 2 corys dying isn't good. Especially since they're the bottom feeders, they're going to be the ones picking up toxins and other nastiness first.
You say your parameters are fine - what are they? Are you using test strips or liquid tests? If you're using test strips, there's a good chance you're not testing for ammonia and that could be what's killing them.
You say your parameters are fine - what are they? Are you using test strips or liquid tests? If you're using test strips, there's a good chance you're not testing for ammonia and that could be what's killing them.