petluvr is right, and I missed it yesterday that you had no nitrates. Just going on cory behavior. But if you're not showing nitrates, you're tank has not finished cycling yet. And that's going to be stressful on the fish. Until you see nitrates, you'll have to keep diligently testing to look for ammonia and nitrite so you don't loose the fish. And whenever you see ammonia or nitrite readings of any sort, you'll need to do a water change to dilute them.
Are you using a liquid test kit for water parameters?